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AcknowledgmentsThe WateReuse Foundation provided substantial funds to support the development of thisreport and the complementing interactive CD on salt management, i.e., the SalinityManagement Guide. Eight other organizations also provided funds: the Bureau ofReclamation, the California Department of Water Resources, the National WaterResearch Institute (NWRI), the Southern California Salinity Coalition, the Central BasinMunicipal Water District, the City of Cerritos, the Los Angeles Department of Water andPower, and the West Basin Municipal Water District. Jeff Mosher of the WateReuseFoundation served as overall project manager at the beginning. Later, after Jeff assumeda position at NWRI, Joshua Dickinson became overall project manager. Jennifer Benderof the Central Basin Municipal Water District served as the day-to-day project manager.Kenneth Tanji of the University of California–Davis is the principal investigator (PI). Dr.Tanji formed an eight-person project team consisting of six co-PIs and a documentationspecialist to address the project objectives.A Project Advisory Committee, assembled by the WateReuse Foundation, reviewed thereport’s contents. The members of the advisory committee were Michelle Chapman,USBR; Victoria Cross, Los Angeles Department of Water and Power; Hoover Ng, WaterReplenishment District of Southern California; Dennis Pittenger, University ofCalifornia–Riverside; and Ken Tate, City of Cerritos. Dr. Tanji was also assisted by apanel of peer reviewers as well. The members of the peer review panel who reviewed oneor more chapters included Takashi Asano, Hossein Ashktorab, Robert Bastian, SharonBenes, Harvey Collins, Larry Costello, James Crook, David Hills, Mike Huck, StephenKaffka, Seiichi Miyamoto, Lorance Oki, Robert Perry, Warren Roberts, and LawrenceSchwankl. iii

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Extended Executive Summary Salt Management Guide for Recycled Waters Used for Irrigation of Landscapes in Coastal Southern California A comprehensive review of the scientific literature has been conducted to consolidate inone document the factors that affect the use of recycled waters for irrigating landscapes inCalifornia’s south coastal region, where potable water is becoming increasingly scarce. Althoughmany opportunities exist for using recycled waters in urban areas and the California RecycledWater Task Force encourages such use, some landscape professionals are reluctant to userecycled water out of concerns that the water may be excessively saline and harmful to landscapeplants. This document presents a Salinity Management Guide for the irrigation of landscapeswith recycled water, including guidelines on evaluating water quality, controlling salinity in theroot zone, discovering the tolerance of plants to salinity and salinity-related effects, anddiagnosing and solving problems that might be encountered in the irrigation of turfgrasses, trees,shrubs, and ground covers. It also includes related aspects of landscape irrigation, includingCalifornia’s Water Recycling Criteria, selecting plants, choosing and using irrigation systems,calculating the water needed by the plants, and mitigating problems with the soil.Title 22 Regulations and Present Use of Recycled Waters for Landscape Irrigation California’s Water Code states that using a potable source of water when nonpotablewater could be safely used instead is a wasteful or unreasonable use of water. The state’s recycledwater regulations are contained in Title 22, Code of Regulations on Water Recycling Criteria.These regulations require tertiary treatment and disinfection of recycled waters used to irrigateparks and playgrounds, school yards, residential landscapes, and golf courses with unrestrictedaccess. This level of treatment, which is aimed at protecting public and ecological health, exceedsthe level of treatment of most potable water supplies and meets the level of treatment required formost wastewaters discharged to waters of the state. The regulations require that recycled waterused to irrigate cemeteries, freeway landscapes, golf courses with restricted access, ornamentalnurseries, and sod farms receive somewhat less treatment, i.e., secondary treatment anddisinfection levels of 2.2 to 23 median counts of total coliform bacteria per 100 mL of water. ES-1

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Of the current 533,000 acre-ft of recycled waters used in California, about 21% is used toirrigate landscapes, mostly turfgrasses in golf courses and lawns. Opportunities to further userecycled waters exist; i.e., recycled waters could be used to irrigate golf courses, lawns, trees,shrubs, ground covers, vines, ornamental plants, and flowers of other landscapes now beingirrigated with potable waters.Significant Constituents in Water Used to Irrigate Most recycled waters do not inherently contain excessively high levels of salinity, eventhough they typically contain about 140 to 400 more milligrams of salts per liter than do thepotable waters from which they originated. The salinity of waters may affect plants due toosmotic effects; i.e., plants must expend more energy to extract water from the soil when thatwater is more saline, and plants may suffer slowed growth, damaged leaves, and death in theseverest cases. Plants have a wide range of tolerance of salinity, and many could be irrigated withrecycled waters. If communities use sodium chloride-based water softeners, the recycled water originatingfrom such communities may contain elevated numbers of sodium and chloride ions. Moreover,use of cleaning agents, such as detergents, may elevate concentrations of boron in recycledwaters. Plants differ in their sensitivity to sodium and chloride ions and boron. Sensitive plantstypically exhibit damaged leaves and, in severer cases, defoliation and death. Excessive levels ofsodium may also cause an imbalance in the mineral nutrition of plants, such as a deficiency ofcalcium. Another significant constituent in recycled waters is nitrogen in the form of dissolvedammonia or ammonium ions and nitrates. The concentration levels of these forms of nitrogen aredependent on the wastewater treatment processes used. Ammonia or ammonium ions in recycledwaters are eventually oxidized into nitrate ions in the soil. Other forms of nitrogen, such asorganic nitrogen and nitrite, occur in smaller concentrations. Nitrogen in recycled water used toirrigate can pose problems if the nitrates not taken up by plants leach below their roots andcontribute to the contamination of underlying groundwater basins. Such leaching of nitrates maybe minimized if the amount of nitrogen in recycled water is taken into account in fertilizerapplications and if less nitrogen-containing fertilizer is consequently applied. The combined effects of salinity and sodicity of irrigation water can affect the soil’spermeability, reducing water infiltration rates and soil permeability. Sodicity is usually evaluatedby its ratio of sodium to calcium plus magnesium, known as the sodium adsorption ratio (SAR),and salinity is typically assessed by electrical conductivity (EC). A moderate SAR and a low level ES-2

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of EC may result in reduced permeability in some types of soil. The detrimental effects of amoderate SAR on a soil’s permeability may be partially overcome by a moderate level of EC.The treatment processes for recycled waters involve the use of additives that elevate the SAR,such as sodium hypochlorite, frequently used to disinfect, and bicarbonate and carbonate from thelime, used to neutralize the water’s pH. Another parameter of sodicity is the residual sodiumcarbonate (RSC), which is the sum of bicarbonate and carbonate ions minus the sum of calciumand magnesium ions. It can be used to evaluate the detrimental effects of sodicity, which cancause the dispersal of organic matter and clays in the soil, resulting in dark unsightly matting onthe turf of golf courses and slower water infiltration into turf soils. Some communities blend several sources of water for potable purposes, such as importedwater from the Colorado River Aqueduct and the California Aqueduct with local surface and wellwaters. These sources contain differing salinities. For example, the Colorado River water containsabout 750 mg of total dissolved solids (TDS)/L, the California Aqueduct water contains about450 mg of TDS/L, and well water contains as little as 200 mg of TDS/L from granitic watershedand alluvium. Blending practices tend to change, according to the demand for water and theavailability of source waters. As a result, water salinity and sodicity may change seasonally withchanges in blending. This situation causes the quality of recycled water to fluctuate. Landscapeirrigators need to keep abreast of these changes in water quality, so as to manage irrigationappropriately. This caution is particularly important when plants in the landscape are sensitive tosalinity and sodicity and when the concentrations of nitrogen are high.Selecting Plants for Coastal Southern California Plants vary in their requirement for sunlight, water, and nutrients, as well as in theirsusceptibility to adverse environmental conditions. Although many plants can tolerate a widerange of conditions, others have distinct preferences for particular climates and soils and do notthrive elsewhere. The natural distribution of plants is determined by the interaction of manyenvironmental factors, including the intensity and duration of sunlight; the temperature; theproperties of the soil; the availability of plant nutrients; the amount of rainfall; the amount andquality of irrigation water; any wind, floods, or fires; and biotic interactions, such as competingwith other plants for space and sunlight, being consumed by plant-eating animals, and beingexposed to disease-causing microbes. Plant ecologists have combined environmental and climatological data to delineate plantenvironment zones or regions. One can determine from this information the type of plants thatwill thrive in these zones. We have reviewed several comprehensive guides for selecting shrubs, ES-3

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trees, and ground covers, including Perry’s 1981 book, Trees and Shrubs for Dry CaliforniaLandscapes, a comprehensive general-purpose guide that lists 360 species of plants, emphasizingspecies that survive with limited water. Part I of his book, titled “Regional Plant Environments,”describes nine plant environments and includes a detailed guide for selecting plants. Part II, titled“Planting Guidelines,” covers appropriate planting concepts within the constraints of function,aesthetics, costs, resources, and maintenance requirements. Perry also wrote a 1992 book,“Landscape Plants for Western Regions,” which builds on his 1981 book and includes sections on“Issues and Goals,” “Regional Characteristics,” “Estimating Water Needs of Landscapes,” andlastly, “Plant Palettes,” for selecting plants that can be combined to achieve visual and aestheticcharacter, along with cultural compatibility. The Sunset Western Garden Book, which is available in more than four editions, isperhaps the best-known and most widely available guide to selecting plants. As with Perry’s 1992book, this book includes a system of climatological zones depicted on maps. A major portion ofthe book is a plant encyclopedia that describes several thousand species of plants used forlandscapes in the western United States. Labadie’s 1978 book, Native California Plants, whichevolved from his years of teaching at Merritt College in Oakland, CA, covers 101 species ofplants that are native to California. A number of brief lists of plants for particular situations, suchas plants that do well in partial shade and plants that tolerate wind, is appended to the book. Lenz and Courley’s 1981 book, California Native Trees and Shrubs, also presents a map-illustrated system of climatological zones. Based on the authors’ 50 years of horticulturalexperience at the Rancho Santa Ana Botanic Garden, the book focuses on trees and shrubs forsouthern California. It contains a comprehensive glossary and a cross-index of the common andscientific names of plants. Lenz also published a book in 1956, Native Plants in California, aspart of a series of papers published by the Rancho Santa Ana Botanic Garden. In that book, henames 102 species of native flora suitable for use by landscape professionals. After Perry and others, we have listed 57 ground covers appropriate for coastal southernCalifornia, identifying them as native or not and the regions where they flourish, i.e., in thecoastal margin, intermediate valleys, coastal foothills, inland valleys, or inland foothills. Thisdocument includes a map of these regions as well as tables identifying well over 300 shrubs andtrees of various heights, including 56 shrubs up to 5 ft tall, 85 shrubs up to 10 ft tall, 96 shrubsfrom 10 to 18 ft tall, 60 trees up to 25 ft tall, 49 trees up to 40 ft tall, and 45 trees that are 40 ft ortaller. Ground covers listed include Little Sur manzanita (Arctostaphylos edmundsii), bearberry(Arctostaphylos uva ursi), coast sagebrush (Artemisia pynocephala), maritime ceanothus ES-4

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floribundus), and Pacific wax myrtle (Myrica californica); and the shrubs laurel sumac (Malosmalaurina) and salal (Gaultheria shallon). Native plants that tolerate sprinkler irrigation by salinewater include the tree Bishop pine (Pinus muricata) and the ground covers Point Reyes creeper(Ceanothus gloriosus) and Monterey manzanita (Arctostaphylos hookeri). We have also summarized notes about selecting plants for certain types of landscapes,including a general landscape design guide, turf and trees for golf courses, turf for playing fieldsand parks, and plants for medians and street sides.Tolerance by Plants of Salinity and Boron An extensive review of the scientific literature was conducted to prepare a list oflandscape plants appropriate for the south coastal region of California, as well as lists oflandscape plants according to their tolerances of salinity and boron. Many earlier studies on thesalt tolerances of plants were conducted in solution cultures or soil pots that were surfaceirrigated, i.e., via the soil. However, much landscape irrigation is conducted via sprinklers, whichwets and exposes leaves to salts in the irrigation water. Fortunately, recent studies regarding thesalt tolerances of plants have involved evaluating the response of plants to salts in both sprinklerirrigation and irrigation via the soil. A book by Perry (1981) of California State University–Pomona identifies 36 salt-toleranttrees and shrubs grown in south coastal California, including 25 rated for their tolerance of saltswhen sprinkler irrigated, 19 rated for their tolerance of salts when irrigated via the soil, and 8rated for their tolerance of both sprinkler irrigation and irrigation via the soil. Salt-tolerant treesinclude the beefwood (Casuarinas spp.), desert gum (Eucalyptus rudis), and coral gum(Eucalyptus torquata) varieties of eucalyptus and the torrey (Pinus torreyana), and aleppo (Pinushalepensis) varieties of pine. Salt-tolerant shrubs include bird of paradise bush (Caesalpinagilliesii), Italian jasmine (Jasminum humile), sandhill sage (Artemesia pycnocephala),pittosporum (Pittosporum crassifolium), and Little Sur manzanita (Arctostaphylos edmundsii). A study at the University of California–Davis by Wu et al. (2001) and Wu and Gao(2005) evaluated the salt tolerances of landscape plants irrigated by sprinklers versus the salttolerances of landscape plants when irrigated via the soil. Three waters of varying qualities wereused: a potable well water with an EC of 0.6 decisiemens (dS)/m, water with an EC of 0.9 dS/mto which 500 mg of sodium chloride/L was added to the well water, and water with an EC of 2.1dS/m to which 1,500 mg of sodium chloride/L was added to the well water. The well waters towhich sodium chloride was added resembled typical recycled water in the San Francisco Bayregion. Sprinkler-irrigated plants were categorized as highly tolerant, tolerant, moderately ES-8

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Clearly, some landscape plants are sensitive to salinity and boron. However, there exists awide array of trees, shrubs, turfgrasses, ground covers, vines, flowers, and ornamental plants thatcould be irrigated with recycled waters containing moderate salinities and moderateconcentrations of sodium, chloride, and boron. Many are listed in this document.Water Quality Guidelines The quality of recycled waters may have measurable or observable effects, some ofwhich are adverse, on plants, soils, and irrigation systems. The assessment and management of irrigation are much more established for agriculturalirrigation than for landscape irrigation, except for the irrigation of turf. Thus, a significant portionof this literature review explored the applicability of the management of agricultural irrigation tothe management of landscape irrigation in terms of evaluating water quality, diagnosingproblems, and implementing management practices. The primary difference between the two isthat the management of agricultural irrigation is aimed at maximizing yield, whereas themanagement of landscape irrigation is focused on maintaining the aesthetic quality andappearance of the landscape. We recommend the Water Quality Guidelines advanced by the United Nations’ Food andAgriculture Organization (FAO) (Ayers and Westcot, 1985). These guidelines for using recycledwater to irrigate croplands and landscapes are used worldwide. A Committee of Consultants fromthe Agricultural Experiment Station of University of California initially proposed these guidelinesafter extensive consultation with the U.S. Salinity Laboratory. The FAO then adopted andextended the guidelines. The FAO guidelines consist of a matrix in which specific irrigation-related problems arealigned vertically and degrees of restriction on use are aligned horizontally. The problems includesalinity, infiltration or soil permeability, specific ion toxicity, and miscellaneous effects. Eachproblem is then associated with particular constituents of water quality, such as salinity by the ECand the TDS; infiltration by the SAR and the EC; specific ion toxicity by the concentrations ofsoluble sodium, chloride, and boron; and miscellaneous effects by nitrogen in the form ofammonia and nitrate, bicarbonate, and the pH. The degrees of restriction on use are categorizedinto none, slight to moderate, and severe, with numeric values or ranges of numeric values foreach parameter identified in cited problems. Though these three categories are somewhatarbitrary since there are no clear-cut specific boundaries to distinguish the categories and since ES-11

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changes occur gradually, the numeric guidelines were based on the collective opinions of soil,plant, and water scientists with extensive research and practical experience. When one uses the FAO’s water quality guidelines, there are a number of caveats andassumptions regarding yield potential, conditions at the site, methods and timing of irrigation, andthe uptake of water by crops. The guidelines cover a wide range of conditions encountered inirrigated agriculture and should be used as an initial evaluation and modified with local expertiseas needed. In particular, the guidelines are not plant specific and may be too restrictive for somesalt-tolerant species of plants and perhaps not restrictive enough for some sensitive species. These guidelines were applied to four representative compositions of recycled waters inCalifornia. These waters had levels of EC ranging from 1.0 to 1.6 dS/m, SAR ranging from 3.4 to4.9, <0 to 1.7 meq of RSC/L, 157 to 185 mg of sodium/L, 188 to 226 mg of chloride/L, 0.4 to 0.6mg of boron/L, 0.2 to 31.3 mg of ammonium/L expressed as nitrogen, and 0.8 to 13.9 mg ofnitrate/L expressed as nitrogen. All of these waters tended to rank in the “slight to moderaterestriction on use” categories, with some exceptions. These exceptions were that three waters fellin the “no restriction on use” category with regard to RSC and boron hazards and that one of themfell in the “severe restriction on use” category due to its elevated concentrations of nitrogen andan RSC value of moderate concern. Certain management practices can help decrease the moderate to severe restrictions onuse. One such practice is to take into account the nitrogen in recycled water and reduce theamount of nitrogen-containing fertilizer applied. Another is to inject an acid or add a calcium-containing amendment to water with a high RSC to prevent organic matter and clays in the soilfrom dispersing and water from poorly infiltrating. Yet another practice is to replace sensitiveplants that may be detrimentally affected by salinity or concentrations of specific ions with moretolerant plants. It is our considered opinion that the FAO water quality guidelines tend to be onthe conservative side. This view was confirmed by a case study of irrigation of turfgrasses withrecycled waters.Salinity Control in the Root Zone The soil is the medium from which plants extract water and essential mineral nutrients. Italso supports the roots of plants. Salts tend to accumulate in the root zone of actively transpiringplants, as water is lost to the atmosphere through transpiration from plants and evaporation fromthe soil, leaving behind the dissolved mineral salts in the soil water. These dissolved mineral saltshave an osmotic effect: as salts increase in the soil, plants must expend greater energy to drawwater from the soil. Also, some ions of these salts, such as sodium and chloride, as well as boron, ES-12

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may accumulate to concentrations in the soil that are high enough to harm plants. Maintaining asalt balance in the root zone is critical for satisfactory plant performance in a semiarid climatewith insufficient rainfall for leaching salts from the root zone. In surface-irrigated soils withunimpeded drainage, salts leach from the upper root zone and accumulate in the lower root zone. Fortunately, most landscape plants are more densely rooted at and near the surface of thesoil, where the soil tends to be least saline. Plants extract soil water from the more saline deeperroot zone only when the soil water that is available in the less saline portions at and near thesurface becomes limited. The extent to which salts accumulate in the lower root zone is regulatedby the leaching fraction (LF), the ratio of the depth of drainage water to the depth of irrigationwater. The depth of drainage water is the irrigation water minus the water lost to the atmospherefrom transpiration by plants and evaporation from the soil. In freely draining soils, acomparatively small depth of drainage may be sufficient to maintain a salt balance in the rootzone. An LF of 0.15 to 0.2 is usually adequate to maintain a salt balance for most agriculturalcrops irrigated with typically saline water. This LF also should be applicable to landscape plantswith a similar range of salt tolerances. Using the FAO approach of computing the accumulation of salts in quartile root zones,i.e., four increments of depth, the principles and applications of steady-state LF were addressedby considering the pattern in which roots extract water, as well as the irrigation water’s LF andEC. The EC of the drainage water past the root zone may be estimated from the ratio of the EC ofthe irrigation water to the LF. Computations can be facilitated with an Excel model that is basedon the assumption that salts are a conservative parameter; i.e., salts are not chemically reactive,such as in mineral precipitation, mineral dissolution, and cation exchange. This model is in anappendix. Also considered were the impact of rainfall on the leaching of salts, any mixedqualities of supply waters, and reclamation leaching with use of a mixing cell Excel model thatincludes the initial salinity of the soil. This model is also in an appendix. More complex aspects of root zone salinity were addressed, including a chemicalequilibrium model (WATSUIT) and its use in assessing the accumulation of salts in quartile rootzones. WATSUIT was also used to assess the precipitation of calcite and gypsum as a function ofthe LF for Colorado River water. Based on these data, a simplified reactive salt accumulationmodel was developed that incorporated prescribed increments of soil depth (typically more thanfour) and their initial concentrations of soil salinity into the mixing cell model. This Excel modelis also in an appendix. Shaw et al. (1995) conducted a case study on the composition of drainage from the rootzone from plots of turfgrass irrigated with potable and recycled waters. These plots were located ES-13

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at the Whispering Palms site on the sandy soils of the San Dieguito River’s flood plains in SanDiego County. Turfgrasses involved in this experiment included cool-season grasses, namely, tallfescue (Festuca arundinacea) and a Kentucky bluegrass (Poa pratensis)-perennial ryegrass(Lolium perenne) mixture, and warm-season grasses, namely, bermudagrass (Cynodon dactylon)and kikuyugrass (Pennisetum clandestinum). Irrigation was scheduled according to the waterbudget method and with the use of real-time data about local weather. Water infiltrated into thesoil from irrigation and rainfall is lost to the atmosphere via transpiration by plants andevaporation from the soil, which is collectively referred to as evapotranspiration (ET). In thestudy conducted by Shaw et al., the ET of the grasses in inches per day approximately equaled 0.6× the reference ET (ETo) for warm-season grasses and 0.8 × ETo for cool-season grasses. Rainfallfrom January 1993 through November 1994 was 25.1 in. The cool-season grasses received 105in. of irrigation, while the warm-season grasses received 84 in. of irrigation. The calculated ETfor cool-season grasses was 74.5 in. and for warm-season grasses was 54.1 in. Irrigation waterplus rainfall minus ET equaled the drainage out of the root zone, which averaged 56 in. for cool-season grasses and 54 in. for warm-season grasses. The LF for cool-season grasses was 0.42. TheLF for warm-season grasses was 0.50. The potable water in the case study had an EC of 1 dS/m, a SAR of 2.7, 0.15 mg/L ofboron, 0.2 mg/L of nitrate expressed as nitrogen, and 0.07 mg/L of ammonium expressed asnitrogen. The recycled water in the case study had an EC of 1.4 dS/m, a SAR of 4.8, 0.5 mg ofboron/L, 11.2 mg of nitrate/L expressed as nitrogen, and 0.2 mg of ammonium/L expressed asnitrogen. Shaw and his colleagues analyzed samples of soil from the root zone, i.e., 0–24 in.below the surface, and samples of soil from below the root zone, i.e., 24–36 in. below the surface.The EC of the extract from a saturated soil paste (ECe) of root-zone samples ranged from 2.7 to3.3 dS/m, and the ECe of samples from below the root zone ranged from 1.7 to 2.5 dS/m. The ECewas only two to three times greater than the EC of irrigation waters because of comparativelyhigh LFs. The plots of turfgrass all received 544 lbs. per acre of nitrogen-containing fertilizer.The nitrogen in the recycled water used to irrigate was equivalent to 225 lbs. per acre. However,nitrate concentrations in the root zone for all treatments were low, ranging from 0.4 to 3.2 mg/Lexpressed as nitrogen, indicating that the grasses extracted much nitrogen. Turfgrasses are knownto be heavy feeders of nitrogen and are often described as luxury consumers of nitrogen. Based on the aforementioned data, the mass loading and emission of nitrogen and TDSwere estimated. The plots irrigated with potable water had a mass loading of 548 lbs. of nitrogenper acre. The recycled-water treatments had a mass loading of 769 lbs. of nitrogen per acre. Themass emission of nitrogen from bermudagrass irrigated with potable water was 47 lbs. per acre ES-14

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and from kikuyugrass irrigated with potable water was 59 lbs. per acre. The mass emission rate ofTDS from bermudagrass irrigated with recycled water was 84 lbs. per acre and from kikuyugrassirrigated with recycled water was 59 lbs. per acre. These rates of mass emission amounted to theleaching of 8 to 13% of the nitrogen from water and fertilizers. The plots irrigated with potablewater had an average mass TDS loading of 5.7 tons per acre, while the plots irrigated withrecycled water averaged 9.2 tons per acre. The mass emission rate of TDS from bermudagrassirrigated with potable water was 8.8 tons per acre (or 150% drained) and for kikuyugrass irrigatedwith potable water was 7.3 tons per acre (or 125% drained). The mass emission rate of TDS frombermudagrass irrigated with recycled water was 7.2 tons per acre (or 77% drained) and fromkikuyugrass irrigated with recycled water was 8.8 tons per acre (or 94% drained). The percentageof salts that drained ranged from 77 to 125%. This range is acceptable, considering that severalsinks and sources of salts within the root zone were not considered in this mass balance, with onlymass inputs and mass outputs calculated. Though the 150% salt leaching appears to beunacceptable, it should be noted that the initial ECe of the soil for bermudagrass irrigated withpotable water was 1.7 dS/m, which is higher than the initial ECe of all the others, which rangedfrom 1.1 to 1.2 dS/m. This case study demonstrated that recycled water can be beneficially used to irrigateestablished turfgrasses, thus conserving potable waters. Relatively few problems were noted.Shaw et al. (1995) had initial concerns about the EC, the SAR, the nitrate, and the boron in therecycled water, but they caused no significant problems. However, Shaw et al. (1995) state thatthe reliability of the recycled water’s quality is a key. Any significant changes in quality shouldbe noted and appropriate management practices taken to avoid problems. In contrast to the situation with established turfgrasses, there can be some concerns whenusing recycled water to establish new turf stands by vegetative parts or seed. Depending upon soiland water salinity levels, newly seeded turf may demonstrate reduced germination percentages,poor seedling vigor, and an overall lower establishment and maturation rate. Cool-seasonvarieties overseeded into established warm-season turf show similar problems that are generallyassociated with higher total salinity and sodium concentrations of the recycled waters. Sod,springs, and stolons can also be affected, showing slower root development and stacking of rootsinto the soil. Higher seeding, springing, and stolonizing rates and planning for a longerestablishment must be considered when using waters of moderate salinities for irrigation ofturfgrasses. Another, more effective approach is to irrigate with nonsaline water until turfgrassstand is well established. ES-15

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Irrigation Systems and Water Requirements of Landscape Plants An irrigation system’s major function is to provide water to plants in a manner suitablefor fostering their growth and performance in the landscape. The system should be able to meetthe landscape’s peak demands for water, apply enough water to leach salts through thelandscape’s root zone, and perhaps be useful in meeting other needs, such as the control of frost.The system should be appropriately and effectively designed, installed, built, operated, andmaintained. The major components for successful irrigation include design, installation andconstruction, operation, and maintenance. A well-designed system contains appropriate irrigationand drainage components for the plants, includes specific construction details and maintenancerequirements, meets regulatory guidelines, and includes a water budget and an irrigation scheduleto establish the landscape, as well as sustain it. The components of an irrigation system typically include the following: a pump whenneeded; a main line and laterals; a flow meter; flow control and pressure-regulating valves; filterswhen needed; parts that apply the water, such as sprinkler heads, bubblers, drip emitters, or driptapes; and a timer to regulate the time and duration of irrigation. The parts of a system thatdistribute and apply recycled water—the pipelines, pumps, valves, sprinkler heads, bubblers,etc.—are all colored purple to clearly distinguish them from parts of systems that distribute andapply potable water. If secondary effluent is used or recycled water that was held in storage pondsbefore application is used, then a filtration system is needed. If acids or other amendments areinjected into the irrigation system, the system’s components must be selected or modified totolerate these amendments. Sprinkler irrigation is the most common method of irrigating with recycled water. Thesprinkler heads may consist of a spray head that delivers water in all directions simultaneously ormay consist of a rotating or impact stream head that directs water over a wider radius than sprayheads do. The sprinkler heads may be those that pop up when operating, or they may be attachedto a riser. Drip irrigation may be placed on a surface, as with a surface drip system, or be placedbelow the surface, as with a buried or subsurface drip system. When one is irrigating landscapes, the differing water needs of the mix of plants in thespecific landscape must be kept in mind. For example, in a landscape consisting of both trees andturf, the trees may need to be irrigated separately with bubblers and drip irrigation because theirwater requirements differ from those of turfgrasses. The installation and construction phase of an irrigation system includes not onlyinstalling the system but coordinating other activities, such as grading the land, preparing the soil,selecting plants, and installing lighting and signage. Operating the irrigation system consists of ES-16

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determining the landscape’s water budget and scheduling its irrigation. Maintenance is essentialfor an efficiently operating system. Proper cultural treatment of plants and other components ofthe landscape not only improves the landscape’s appearance and value but can also affect the useof water in the landscape. The ET of crop plants has been widely investigated and known, but such is not the casewith the ET of landscape plants, except for that of turfgrasses. In agriculture, the ET of crops(ETc) is estimated by a number of methods. Weather-based estimates of ET are obtained bymultiplying the reference ET (ETo) by the crop coefficient (Kc). The monthly Kc for cool-seasonand warm-season turfgrasses in California is available in this document. The Kc for establishedtrees and shrubs is also available in this document. The daily ET may be estimated and compiledas weekly, monthly, or seasonal ET by using data from the California Irrigation ManagementInformation System (CIMIS), a network of over 120 stations strategically placed throughout thestate providing hourly and daily ETo that is electronically based on the amount of sunlight, thetemperature, the relative humidity, wind speed, etc. A few water agencies have installed their ownweather stations. Some irrigators may use historic ETo values instead of real-time data. Estimating coefficients for other types of plants for landscapes, especially heterogeneousmix of plants, is more difficult than for turfgrasses. Research-based data regarding the waterneeded by plants in landscapes with a mix of plants are limited. Plant species with differing needsfor water exist, and those needs are influenced by their location in the landscape and theirinteraction with the surrounding environment. This complexity severely limits the ability toaccurately estimate water needs using the ETo-Kc approach. Despite these limitations, severalapproaches for estimating water needed by a landscape have been proposed. One popular method, the Water Use Classification of Landscape Species (WUCOLS)(University of California Cooperative Extension and California Department of Water Resources,2000), introduces a landscape coefficient (KL) adjusted to take into account for differences inlandscape species (Ks), plant density (Kd), and microclimate (Kms). Though this method takes intoaccount factors that affect the KL, quantitative data are not readily available, and thus ETo × KLproduces a rough initial estimate of ET that will need to be adjusted after the initial estimates areobtained. Procedural guidelines to assign numerical values for Ks, Kd, and Kms for high,moderate, low, and very low values for landscape coefficient factors are outlined in WUCOLSIII. Since California’s climate varies substantially, hundreds of plant species are evaluated for sixregions (climatic zones). It is expected that, after extensive application and testing, the WUCOLSapproach will become more reliable. ES-17

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With many recycled waters, irrigating beyond what is needed for ET is typically requiredfor leaching salts. The irrigation system’s uniformity of application is another important factor foradding water beyond what is needed for ET. The irrigation system’s uniformity of distributionshould be considered in the total water applied, too. It should be noted, though, that runoff fromareas irrigated with recycled water is prohibited. Uniform distribution of applied water is extremely important for root zone salinitymanagement in golf and sports turf. Achieving a uniform application will maintain a uniformwetting front when one is leaching salts through the soil profile and prevents the development ofexcessively wet or dry area associated with poor root distribution. In golf or sports turf situations,this precaution not only impacts aesthetics but also safety implications (e.g., firm footing) andcustomer satisfaction by providing a dry playing surface. Scheduling irrigation involves calculating when and how much to irrigate. When toirrigate is determined by one of several methods, including the flexible or soil water depletionmethod, the fixed calendar method, or the soil moisture sensor method. How much to irrigate isdetermined by estimates of the plant’s ET, the irrigation system’s rate of application, and thesystem’s uniformity of distribution. A number of water calculators are available to scheduleirrigation, including some from local water districts and other local agencies. A properly designedand well-managed irrigation system will provide optimal amounts of water to landscape plants,except perhaps when a mixture of species needs to be irrigated.Soil Problems and Management Options As previously discussed, the quality of recycled water may affect plants and soils.Specifically, salinity of water and specific ions in water may affect plants and the soil’spermeability. There are other aspects of particular note when irrigating a landscape with recycledwater. One such aspect is the salinity of the soil, denoted by the EC of an extract of saturatedsoil paste (ECe) that may affect the growth of plants, and the sodicity of the soil, indicated by theexchangeable sodium percentage (ESP) or SAR. Soils are considered nonsaline if the ECe is lessthan 4 dS/m, the ESP is less than 15%, and the pHs (pH of the saturated soil paste) is less than8.5. Saline soils have an ECe of more than 4 dS/m, an ESP of less than 15%, and pHs of less than8.5. Sodic soils have an ECe of less than 4 dS/m, an ESP of greater than 15%, and pHs of morethan 8.5. Saline-sodic soils have an ECe of more than 4 dS/m, an ESP of more than 15%, and pHsof less than 8.5. ES-18

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Measuring the salinity of water in terms of the EC or TDS is quite straightforward, butmeasuring the salinity of soil is more challenging due to its dynamic nature. The salinity of soilchanges over time, with irrigation and rainfall replenishing water in the soil and evaporation andtranspiration depleting it. Moreover, dissolved mineral salts are highly mobile in the soil due totheir transport by the flow of water. Thus, the roots of plants are exposed to temporal and spatialchanges in soil salinity. Such changes pose a challenge in measuring soil salinity. Methods formeasuring soil salinity include sampling the soil and analyzing the ECe in a laboratory; measuringthe salinity of soil water in terms of its electrical conductivity, i.e., the ECsw, with the use ofdevices such as an EM-38 electromagnetic probe or a time domain reflectrometry (TDR) probe;and using ceramic suction probes to collect soil water from moist soils and then measuring the ECas ECsw. As pointed out earlier, soil salinity may be controlled by the LF. When the plant’sthreshold salt tolerance is known, the average salinity of the root zone may be regulated with theleaching requirement (LR), which includes enough water to meet the plant’s ET and to leach saltsyet remain within the plant’s threshold salt tolerance. As previously discussed with well-drainedsprinkler-irrigated soils, the root zone at the surface is where salts are leached and the lower rootzone is where salts accumulate. Fortunately, most plants have the densest roots in the upper rootzone nearest the surface, where it is least saline, and the sparsest roots in the lower root zone,where it is most saline. Drip irrigation results in a different pattern of salt distribution. The wettedzone of drip-irrigated soils is somewhat ellipsoidal in shape, with salts tending to accumulate inthe outer edges of the wetted perimeter. After prolonged drip irrigation, salts may accumulatebetween drip emitters to levels that are detrimental to plants and may need to be leached with theuse of sprinkler irrigation. Heavy rainfall on salinized drip-irrigated soils will redistribute the saltsvertically and horizontally, affecting salt-sensitive plants. To prevent such redistribution of saltsby reducing the lateral flow of salts, operation of drip irrigation is recommended during rainfall. A high ESP and SAR in the soil will adversely affect the structure of the soil, especiallyat the surface, causing aggregates of soil to break down and clays and organic matter in the soil todisperse. This process, in turn, reduces the rate at which water infiltrates the soil. Excess ESP iscommonly ameliorated by adding calcium amendments, such as gypsum (CaSO4·2H2O), to thesoil or into the irrigation water. As acids react with soil calcite (CaCO3) to produce solublecalcium, sometimes acids, such as sulfuric acid, and acid-forming amendments, such as elementalsulfur, are used to reduce ESP. Slow water infiltration may also be caused by surface crusting insome soils, which results from the beating action of raindrops and the spray of water from ES-19

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sprinklers, or from the compaction of soil from vehicular and foot traffic, especially in a clayey,moist soil. Maintaining adequate plant nutrition is important to keep plants healthy and attractive.Plants need 17 essential mineral nutrients. Three of these—carbon, hydrogen, and oxygen—arereadily available from the atmosphere and water. Another three—nitrogen, potassium, andphosphorus—are known as primary nutrients because plants need them in large amounts. Threemore—calcium, magnesium, and sulfur—are secondary nutrients and required by plants in lesseramounts. The remaining eight elements are required in trace amounts and are known asmicronutrients. They are zinc, iron, manganese, copper, boron, molybdenum, chlorine, andnickel. When these nutrients become less available to plants, visible symptoms of deficiency areoften noted. Symptoms include discolored leaves, spotted leaves, dead leaf margins, and injuredbuds. It should be noted that some symptoms of deficiency may look like symptoms of anotherdeficiency. For example, symptoms of a deficiency of manganese closely resemble symptoms ofa deficiency of iron or symptoms of damage from the pre-emergence application of herbicides.The location of symptoms on the plant can be very useful in diagnosing deficiencies. Forexample, symptoms of deficiencies of the three most commonly limited nutrients—nitrogen,phosphorus, and potassium—become noticeable on older leaves first, while symptoms ofdeficiencies of sulfur, iron, and zinc first become apparent on newly emerging leaves andsymptoms of deficiencies of boron and calcium manifest early on as dead buds or the dieback ofgrowing tips. Landscapes contain a wide range of plant species, and therefore, it is not surprising thatmineral nutrient requirements can vary widely as well. For instance, turfgrasses require a largeamount of nitrogen, while many species of flowers require higher proportions of phosphorus andpotassium. Inorganic and organic fertilizers can be added to nutrient-deficient soils. The grading of land in landscapes may result in the loss of topsoils, if topsoils areremoved with cut portions, leaving behind infertile soil, or if infertile soils are used as fill soils,e.g., if infertile fill soil from a construction site is used to convert a landfill to a golf course.These infertile-soil landscapes established on sandy and gravelly soils, as in river floodplains orstream channels, typically require more fertilization.Diagnosing and Solving Problems The last chapter of this document covers the diagnosis of problems and suggestedmanagement solutions. The chapter focuses on salinity-related problems encountered in ES-20

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landscapes, but since such problems should not be viewed in complete isolation, it also includesother landscape problems. A problem encountered in landscapes may have multiple abiotic andbiotic causes; thus, accurately diagnosing and appropriately solving a problem may bechallenging. Sources of abiotic stress that may cause or contribute to the injury or disease of aplant include salinity, deficiencies and excesses of minerals, extremes of moisture andtemperature, wind, air pollutants, and drift of herbicide. Sources of biotic stress that may cause orcontribute to the injury or disease of a plant include insects, mammals and birds, bacteria, fungi,nematodes, and viruses. These problems need to be addressed in a timely and comprehensivemanner to avoid high maintenance costs and sustain the quality of landscapes. This last chapter,drawing upon information from previous chapters, summarizes irrigation and drainage problemsand the abiotic factors that cause problems for plants. We consider such problems related to irrigation and drainage as plants suffering fromwater stress, which could be caused by insufficient irrigation and may be solved by increasing theduration and/or rate of irrigation enough to satisfy the plant’s ET; the presence of dry or wetareas, which could be caused by poor uniformity of irrigation and may be solved by changingthe spacing of lateral lines and sprinkler heads or nozzles to improve the uniformity of irrigation;excessive ponding, which could be caused by water with a high SAR and a low EC and may besolved by adding gypsum to the soil; waterlogging, which could be caused by compacted soil andmay be solved by reducing foot and machinery traffic; and runoff, which could be caused by theslow infiltration of water through the soil and may be solved by decreasing the rate and/orduration of irrigation. We also consider such problems involving turfgrasses and lawns as localized dry and wetspots, which could be caused by compacted soil at the surface and may be solved by core-aeratingthe soil; spotty bare spots with salt crust, which could be caused by an excessively saline soil andmay be solved by conducting localized leaching to remove salts; bare spots with dispersedorganic matter, which could be caused by an excess of RSC in the water and may be solved byinjecting acids into the source water; uniform abnormal yellowing of leaves, which could becaused by a deficiency of nitrogen and may be solved by applying nitrogen-containing fertilizersand improving drainage; unusual yellowing of younger leaves, which could be caused by adeficiency of iron and may be solved by applying iron chelate or other iron-containing fertilizers;the dark green discoloration of older leaves, which could be caused by a deficiency of phosphorusand may be solved by applying appropriately broadcasted phosphorus-containing fertilizers; andleaf rolling, which could be caused by a deficiency of potassium and may be solved by ES-21

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broadcasting potassium-containing fertilizer and incorporating it into the ground as much aspossible. We furthermore consider such problems involving trees and shrubs as atypicallyyellowed and prematurely dropping leaves, which could be caused by excessive irrigation and/orpoor drainage and may be solved by decreasing irrigation and improving drainage and aeration;abnormally light green and short needles on conifers, which could be caused by a deficiency ofnitrogen and may be solved by applying a nitrogen-containing fertilizer or improving therestricted growth of roots; the bronzing of lower leaves with purple or brown spots, which couldbe caused by a deficiency of phosphorus and may be solved by applying a phosphorus-containingfertilizer and checking for damages from the use of herbicide; deadened tips of needles inconifers, which could be caused by a deficiency of potassium and may be solved by applying apotassium-containing fertilizer; uncharacteristically yellowish and undersized new leaves withgreen veins, which could be caused by a deficiency of iron and may be solved by adding acidicamendments or iron chelates to lower the soil’s pH; discolored leaves, which could be caused bysunburn or scalding and may be solved by selecting more sun-tolerant plants; trees appearingstressed by lack of water, with dropping leaves and injured bark and trunk, which could be theresult of wind damage and may be solved by selecting wind-tolerant plants and providing windbreaks; and unusually yellowish to brown leaves or needles, which could be caused by airpollution and may be solved by selecting more ozone-tolerant plants. The appendices, in addition to the Excel models, contain a glossary, acronyms andabbreviations used in this report, and conversion factors for SI (Système International) and non-SIunits, chemical units and other useful conversions, a table for field capacity and available soilmoisture as a function of soil texture, and a subject index. This Salt Management Guide will be heavily cited and attached in the forthcominginteractive CD, the Salt Management Guide for Landscape Professionals. ES-22

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Chapter I. Introduction K. Tanji and B. Sheikh California will need to improve its efficiency of water use, both in the agricultural andurban sectors, to meet its water needs by 2030. This exigency is indicated by the most recentwater plan of the state (California Department of Water Resources, 2004). The plan furthersuggests that California water providers will find it advantageous to recycle more water than theycurrently do. Recycled water produced from wastewater already treated to a fairly high level,typically tertiary (secondary treatment, filtration, and disinfection), can be used in manynonpotable applications and, therefore, can help reduce the overall demand for fresh water. Currently, California’s agricultural, industrial, and urban sectors use a total of about530,000 acre-ft of recycled municipal wastewater per year. About 46% is used to irrigateagricultural crops. About 21% is used for landscape irrigation and about 14% for groundwaterrecharge. The rest—19%—goes to various uses, such as cooling water for oil refineries andpower plants and flushing toilets and urinals, as well as to environmental enhancements, such assupplying water for wetlands and ponds, including reflecting ponds. In urban areas, recycledwater often is used to irrigate golf courses, commercial and residential landscapes, plant nurseries,parks and greenbelts, school yards and playing fields, and highway medians and margins.According to a recent survey, 409 parks or playgrounds and 295 schools’ grounds in Californiaare irrigated with recycled water (Crook, 2005). By the year 2030, it is estimated that an additional 1.2 million acre-ft of recycled waterwill be available annually. That water, if used, could free up enough fresh water to meet thehousehold water needs of 30 to 50% of the 17 million additional people who will live inCalifornia in 2030 (California Department of Water Resources, 2004). The expanded use ofrecycled water for landscape irrigation is of especially high priority in south coastal California(the Los Angeles-to-San Diego corridor) in order to help alleviate current and future shortfalls ofpotable water. Recycled water is used for many nonpotable uses in California at the present time.Though many additional opportunities for using recycled water in California’s urban areas existand though such use is encouraged by the state (California Recycled Water Task Force, 2003),some landscape irrigators are reluctant to use recycled water. Some do not fully understand thatrecycled water can be safe and suitable for irrigating landscapes. Some believe that recycledwater may be excessively saline and therefore harmful to landscape plants. I-1

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To help foster a broader acceptance of recycled water, the Central Basin Municipal WaterDistrict, the WateReuse Foundation, the California Department of Water Resources, and severalother institutions recently teamed up to begin informing the public and members of the landscapeindustry about the utility of recycled water. Part of that program involves developing aninteractive, CD-based salt management guide for landscape professionals. Another part of theprogram involves outreach—developing and publishing an educational brochure. A third partinvolves researching the state of knowledge and publishing a literature review summarizing whatis known at present regarding the factors that control the need for salt management when one isirrigating a landscape with recycled water. This document comprises the literature reviewcomponent of the program. It should be noted that, although there are several indirect potablereuse projects involving groundwater recharge, this review does not address potable reuse orpotential health-related groundwater contamination resulting from irrigation with recycled water.It also does not cover the public health aspects of using recycled water, as the authors do not haveexpertise on this topic. And, except in passing, this review does not address the effect of irrigatinglandscape plants with recycled water on the regional salinity of underlying groundwater basins,since regional salt balance in southern California is a complex topic that will require additionalresearch, including three-dimensional modeling coupling unsaturated and saturated zones fortransport of water and salts in site-specific hydrogeologic formations.ReferencesCalifornia Department of Water Resources. 2004. The California Water Plan. CaliforniaDepartment of Water Resources, Sacramento, CA.California Recycled Water Task Force. 2003. Final Report: Water Recycling 2030.Recommendations of the California Recycled Water Task Force, June 2003. CaliforniaDepartment of Water Resources, Sacramento, CA.Crook, J. 2005. Irrigation of Parks, Playgrounds, and Schoolyards with Reclaimed Water: Extentand Safety. WateReuse Foundation, Alexandria, VA. I-2

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Chapter II. Present Status: Potential Benefits of Irrigating Landscapes with Recycled Water, Current Use of Recycled Water, and Regulations K. Tanji and B. Sheikh II.A. Potential Benefits of Using Recycled Water II.B. Current Uses of Recycled Water II.C. California’s Relevant Regulations II.D. References This chapter summarizes the potential benefits of using recycled water, given the increasing scarcity of potable water, as well as the current uses of recycled water for irrigating landscapes in California. The chapter also summarizes the state’s regulations governing use of recycled water. II.A. Potential Benefits of Using Recycled Water Substituting recycled water for valuable and scarce potable water often serves to augment supplies of fresh water. Communities and water purveyors also may benefit in other ways, too. As mentioned in several publications (California Department of Water Resources, 2004; California Recycled Water Task Force, 2003; Sheikh et al., 1998; WateReuse Foundation, 2003), the benefits of using recycled water include the following: · When uncertainties exist with a supply of traditional (potable) water, the use of recycled water for such nonpotable applications as landscape irrigation can help reduce the demand on a water system, thereby increasing the supply of available water and improving the reliability of its supply. · Augmenting a water system with recycled water can, in some situations, decrease the diversion of fresh waters from sensitive ecosystems. · Recycling treated wastewater reduces the discharge of effluent to sensitive environments and protects the quality of surface water and groundwater. Furthermore, recycled water may be used to enhance and create wetlands and riparian habitats. II­1

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· Using recycled water may reduce the costs of wastewater treatment and disposal. It may also provide other economic benefits to dischargers and, indirectly, to businesses and the public. · In communities that recycle water, water purveyors may be able to “bank” a portion of their imported water during average and above­average water years or to reserve some of the imported water for use during dry years. · The use of recycled water, obtained from a local source, often partially offsets the need to import water. That strategy, in turn, reduces the need for pumping and other energy­ consumptive activities associated with importing water. II.B. Current Uses of Recycled Water 0B Recycled water has been used in California since the late 1800s (California Department of Water Resources, 2004). Guidelines and regulations directed at public health protection with regard to water reuse have been in effect since the early 1900s. Use of recycled water has increased during the past several decades, as water agencies strove to meet shortfalls in supplies of potable water caused by drought or population growth and concurrent increases in the demand for water. Currently, with California’s population continuing to increase by approximately 500,000 people per year and with additional new supplies of water virtually nonexistent or increasingly expensive to develop, recycled water could be considered the fastest­growing supply of water available (California Department of Water Resources, 2003). The Office of Water Recycling at the California State Water Resources Control Board recently surveyed water users to determine the amount of municipal wastewater being recycled and the types of recycled water use (California State Water Resources Control Board Office of Water Recycling, 2002). The survey determined that, as of 2002, approximately 525,000 acre­ft of wastewater was being reclaimed and recycled in California each year. At that time the survey was conducted, 48.5% of the total amount of recycled water used in the state was used for agricultural irrigation, 21.1% for landscape irrigation, 9.3% for groundwater recharge, 7.8% for recreational impoundments, 4.9% for seawater barriers, and 11.1% for other uses. Note that these figures refer to direct and intentional use and exclude indirect or incidental reuse such as the disposal of treated wastewater effluent into rivers and streams and subsequent diversion of the river water by downstream water users. According to data compiled by the Office of Water Recycling in 2003 (Table II.B.1), the proportion of total recycled water used for landscape irrigation in southern California ranges II­2

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Table II.B.1. Use of Recycled Water in Selected Service Areas in California (from California State Water Resources Control Board Office of Water Recycling, 2002). Purpose Amt. of water used in: Los Angeles Santa Ana San Diego region San Francisco Bay region region region Agricultural irrigation 3,752 acre­ft/year or 30,795 acre­ft/year 5,033 acre­ft/year or 8,318 acre­ft/year 2% or 37% 16% or 28% Landscape irrigation 26,229 acre­ft/year 28,135 acre­ft/year 24,191 acre­ft/year 10,114 acre­ft/year or 17% or 34% or 78% or 34% a Groundwater 46,247acre­ft/year or 0 acre­ft/year 286 acre­ft/year 0 acre­ft/year recharge 30% or 1% Seawater barrier 10,651acre­ft/year or 15,000 acre­ft/year 0 acre­ft/year 0 acre­ft/year 7% or 18% Other uses 65,437 acre­ft/year 97,20 acre­ft/year or 1,445 acre­ft/year or 11,087 acre­ft/year or 43% 12% 5% or 38% Total recycled water 152,316 acre­ft/year 83,650 acre­ft/year 30,955 acre­ft/year 29,519 acre­ft/year a Currently, greater than 0%. from 17% for the Los Angeles region to 78% for the San Diego region. From these figures, it is evident that opportunities exist to further use recycled waters to irrigate landscapes. II.C. California’s Relevant Regulations California’s regulations governing the use of recycled water are known as Water Recycling Criteria and are found in Title 22, Division 4, Chapter 3, of the California Administrative Code and are often simply referred to as Title 22, Code of Regulations on Water Recycling Criteria (California Department of Health Services, 2001). According to Section 13550 of the California Code, using a potable source of water—for example, to irrigate cemeteries, golf courses, landscaped areas along highways, greenbelts, and parks and playgrounds—is a wasteful or unreasonable use of water if reclaimed water is available that meets certain conditions (State Water Resources Control Board, 2000). These conditions include the following (Crook and Surampalli, 1996): · The source of recycled water is of adequate quality for the proposed uses and available for such uses. · The recycled water may be furnished for these uses at a reasonable cost comparable to, or less than, the cost of potable water. · After concurrence with the California Department of Health Services, the use of recycled water from the proposed sources will not be detrimental to public health. II­3

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Secondary treatment of wastewater includes removal of biodegradable organic matter in solution or suspension and suspended solids (Tchobanoglous et al., 2003). Typically, conventional secondary treatment also includes disinfection. Tertiary treatment of wastewaters includes removal of residual suspended solids after secondary treatment by usually membranes, granular medium filtration or micro­screen. Disinfection is also a part of tertiary treatment (Tchobanoglous et al., 2003). · The proposed uses of recycled water will not adversely affect downstream water rights, will not degrade water quality, and is determined to be not injurious to plants, fish, and other wildlife. Before recycled water can be used to irrigate a landscape, the water must be treated to certain secondary and tertiary levels (Table II.C.1). All recycled water used for landscape irrigation must be disinfected. Water that has not been disinfected is deemed unacceptable for any type of landscape irrigation. For irrigation of cemeteries, freeway margins, sod farms, and other such places where public contact with irrigation water is unlikely, the requirements for treating recycled water are less stringent than those for irrigation with recycled water of public use lands that have unrestricted access, such as golf courses, parks, and playgrounds. Undisinfected, secondarily treated recycled water is acceptable for ornamental nursery stock and sod farms, provided no irrigation with recycled water occurs for a period of 14 days prior to harvesting, retail sale, or access by the general public. As pointed out by Levine and Asano (2004), new or advanced types of treatment processes eventually may be necessary to respond to chemicals that newly emerge and become introduced into municipal wastewater—for example, residues from pharmaceuticals and personal care products. However, these newly emerging chemicals appear not to have an adverse effect on landscape plants. Levine and Asano further assert that recycling treated wastewater is increasingly becoming a necessity. Especially in arid and densely populated areas, such as the Los Angeles basin, where freshwater resources are becoming scarce, recycling wastewater and prioritizing its reuse are essential activities if water supplies are to be truly sustainable in the future. II­4

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Chapter III. Water Quality Guidelines for Recycled Water Use for Landscape Irrigation K. Tanji, S. Grattan, D. Shaw, and B. Sheikh III.A. Water Quality Constituents of Concern in Irrigation Water III.A.1. Salinity or Osmotic Effects III.A.2. Specific Ion Toxicity III.A.3. Sodicity and Soil Permeability III.A.4. Other Constituents of Concern III.B. Water Quality Guidelines III.B.1. Early Water Quality Guidelines III.B.2. Current Guidelines III.B.3. Caveats and Assumptions for Using Current Guidelines III.C. Quality of Recycled Waters III.C.1. Representative Composition of Recycled Waters III.C.2. Evaluation of Representative Recycled Waters Using FAO Guidelines III.D. A Case Study: Use of Potable and Recycled Waters on Turfgrasses at Whispering Palms Site III.E. References Most recycled waters do not inherently contain excessively high levels of salinity, though they typically carry about 150 to 400 mg of salts/L more than does the potable water from which they originated. Given a supply of potable water of low to moderate salinity, the recycled water resulting from it would still be quite suitable as irrigation water for all practical purposes, under most conditions. Evaluating the suitability of waters for irrigation requires a broad understanding of water quality characteristics and interactions with plant, soil, and irrigation management systems. This chapter identifies key water quality parameters and describes how they are interpreted for suitability or to serve as water quality guidelines. Since water quality assessment and management in irrigated agriculture are much more established than in landscape irrigation except for turf irrigation, a significant portion of this chapter involves what is applicable to irrigated crop production that is also applicable to irrigated III­1

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landscape management in terms of evaluating water quality, diagnosing problems, and choosing management strategies. It should be noted, however, that a major difference in evaluating the suitability of waters for irrigating agricultural crops and waters for irrigating landscape plants is that the former is based on harvested crop yield, while the latter is based on aesthetic quality or appearance. Discussions of the quality of irrigation water in the agronomic literature, which refer to water that is not recycled, are equally applicable to recycled water, as discussed in the following section. The principal book references on this topic include Ayers and Westcot (1985), Pettygrove and Asano (1986), and Tanji (1990). III.A. Water Quality Constituents of Concern in Irrigation Water Recycled water contains dissolved mineral salts, nutrients, and residues of chemicals used in the treatment and disinfection of the recycled water. Though all water supplies contain dissolved mineral salts, the dissolved­mineral content of recycled waters primarily depends on the quality of the source water supply and the incidental addition of a small amount of salts— typically from about 100 to 400 mg/L—stemming from the water’s use for municipal and industrial purposes. A larger amount of salts will accrue if water softeners containing sodium chloride (NaCl) are used extensively in the community that contributes wastewater flowing to the wastewater treatment plant. Nutrients contained in recycled water include ammonia, ammonium ions, nitrates, and phosphorus. The concentrations of these nutrients will vary depending on the extent of wastewater treatment provided. The principal constituents of concern with regard to the quality of recycled water for irrigation are the following: salinity, which contributes to osmotic effects that affect the availability of soil water to plants; specific ions toxic to sensitive plants—for example, sodium, chloride, and boron; and the combined effects of sodicity and salinity, which affect the rate at which water infiltrates the soil surface and the permeability of the soil profile. Other constituents of concern include nitrogen, bicarbonates, residual chlorine, and constituents that may cumulatively clog the small orifices of sprinkler irrigation systems. It should be noted that those parameters of water quality that affect human health, such as pathogenic bacteria, protozoa, and viruses, and those that affect the environment, such as dissolved oxygen and oxygen­demanding organics, are not addressed in this document. III­2

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III.A.1. Salinity or Osmotic Effects The salinity of water affects plants due to osmotic effects: plants must expend more energy to extract soil water from saline soil solutions than from nonsaline soil solutions. A widely used indicator of the salinity hazard posed by waters to plants is electrical conductivity (EC, specific conductance), a lumped salinity parameter. Water salinity can be readily measured as EC having units of decisiemens per meter (dS/m), equivalent to millimhos per centimeter (mmhos/cm), and millisiemens per centimeter (mS/cm) in more saline waters and microsiemens per centimeter (mS/cm), equivalent to micromhos per centimeter (μmhos/cm) in less saline waters. EC is a readily obtained measure of how easily electric current is conducted by charged + ions present in the water. Waters contain positively charged ions—major cations such as Na , Ca , Mg , K , NH4 + , and H —and negatively charged ions—major anions such as Cl , HCO3 2− , 2+ 2+ + + − CO3 2­ , SO4 2− , and NO3 − . The higher the salt content, the greater is the EC. Since water is an electrolyte and since electrical neutrality prevails in nature, the milliequivalent­per­liter (meq/L, based on equivalent combining weight) concentration of cations is balanced by the meq/L concentration of anions. EC is the lumped salinity parameter that is preferred for use with water used to irrigate plants because EC can be readily related to osmotic pressure (OP in atmospheres = EC in dS/m ´ 0.36), affecting the availability of soil water to plants. Another lumped salinity parameter for waters is total dissolved solids (TDS, sometimes referred to as dissolved residues). Obtained labor intensively in a laboratory, TDS is a parameter of capacity expressed in mass per unit volume: milligrams per liter (mg/L) or parts per million (ppm) on a volume basis for fresh waters and recycled waters and gallons per liter (g/L) or parts per thousand (ppt) on a volume basis for saline waters, such as seawater. The conversion of EC to TDS varies, depending on the composition of cations and anions and the overall concentration of + − dissolved salts. For example, a salt solution dominated by Na and Cl ions has a higher EC than do Na and SO4 2− ions (or Na and HCO3 − ions) of equal meq/L concentration, because a Cl ion + + − conducts more electricity than do SO4 2− and HCO3 − ions. Nevertheless, TDS in mg/L may be estimated from EC in dS/m by multiplying EC by a rule­of­thumb factor of 640 (a factor of 735 appears to fit better for waters of mixed composition such as Colorado River water). For ECs greater than about 5 dS/m, a conversion factor of 800 is suggested to convert EC into TDS. A third salt concentration unit is tons of salt per acre­foot (ac­ft) of water, which can be estimated from TDS (tons salt/ac­ft = TDS in mg/L ´ 0.00136) or from EC (tons salt/ac­ft = EC in dS/m ´ 0.87; sometimes, a factor of 1.00 is used instead of 0.87). III­3

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A fourth salt concentration unit less frequently used in irrigation practice is total soluble cations or total soluble anions in meq/L. Analytical chemists check their water analyses in meq/L by balancing the sum of cations and anions. If there is a substantial imbalance, they reanalyze the water. Total soluble cations (or total soluble anions) in meq/L may be obtained by multiplying EC in dS/m by 10. The accumulation patterns of salt in irrigated soils depends on the irrigation systems used and the amount of water applied that exceeds crop water demands (Figure III.A). When water is uniformly applied across the irrigated land, as in sprinkler and border irrigation, the surface soil depths become the zone of salt leaching and the bottom soil depths become the zone of salt accumulation. The extent of salt accumulated in the bottom of the root zone depends on the leaching fraction (LF, namely, the ratio of drainage out of the root zone to infiltrated water). The higher the LF, the less salt is accumulated in the soil. When water is applied by furrow irrigation, salts increase with soil depth in the bottom of the furrow while the beds of the furrow tend to accumulate salts. When water is applied by drip irrigation, salts tend to accumulate concentrically around the wetted perimeter of the zone irrigated. Soils may contain soluble minerals that, when chemically weathered, contribute to the overall salinity in the soil solution. Soil minerals such as calcite (CaCO3) and feldspars (sodic­, calcic­, and potassium silicates) have low solubilities and contribute relatively little to soil salinity, while minerals such as gypsum (CaSO4∙2H2O) have higher solubilities and may contribute significant concentrations of Ca and SO4 ions. The solubility of gypsum in pure water + 2+ is about 2,600 mg/L and much higher in the presence of Na and Mg ions (Tanji, 2002). Other more highly soluble evaporite minerals, such as sodium chloride, sodium sulfate, and magnesium sulfate, are sometimes present in strongly salt­affected soils. These highly soluble salts are readily leached by rainfall and irrigation into deeper zones, sometimes beyond the root zones. The salinity parameter of interest on plant performance is EC to assess osmotic effects. Osmotic effects on plants are reflected by stunted growth, chlorosis, and wilting in some cases and death in the most severe cases. Plants vary in their tolerance to salts (osmotic effects) as indicated in Chapter V of this document. Salt tolerant plants expend less metabolic energy to adjust to a saline environment than do more salt­sensitive plants (Lauchli and Epstein, 1990). III­4

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III.A.2. Specific Ion Toxicity Some plants may be sensitive to specific ions such as Na , Cl- , and B, the last usually in + the form of undissociated boric acid or H3BO3 at pH in the neutral range. These negative impacts are collectively known as specific ion toxicity (Chapter V). As pointed out above, the use of water softeners increases the concentration of Na and Cl- ions in recycled waters. Also, a few + communities use potassium chloride (KCl) instead of sodium chloride (NaCl) in their water softeners, increasing the concentration of Cl- ions in the recycled water. Use of boron­containing household cleansers may elevate concentrations of boron in recycled waters. The symptoms of specific ion toxicity include chlorosis and necrosis on marginal edges of leaves, necrotic spots on leaves, interveinal chlorosis on leaves, damage to growing tips, and death in the severer cases. Though some specific ions, particularly boron and chloride, are essential to plant growth in low concentrations, the range in concentration between essential and toxic is narrow in sensitive plants. Plants also vary in their tolerance to specific ions (Chapter V). Woody plants (e.g., trees and shrubs) tend to be more severely affected by specific ions than do annual plants, since specific ions may be translocated and accumulated over time in roots, trunks, leaves, and growing tips. Annual plants may suffer from specific ion toxicity if the water contains elevated concentrations of such ions and accumulates to a sufficient degree during the shorter growing period of the plants. Toxicity to specific ions, especially Cl- and Na , can also occur in annuals + and perennials from direct absorption through the leaves when wetted by sprinkler systems. Visual diagnosis of foliar damage due to specific ions may be compounded by osmotic effects (salinity). As waters increase in salinity, specific ions also tend to increase in concentration, especially Na and Cl- ions. Thus, osmotic effects and specific ion toxicity + frequently cannot always be clearly differentiated. In such cases, chemical analyses of leaf tissues for specific ions and salinity may more accurately reveal the cause(s) of plant damage or poor performance. III.A.3. Sodicity and Soil Permeability + Excess Na in waters may impact mineral nutrition in plants, causing Na­induced calcium deficiency and specific ion toxicity and affecting soil permeability and rates of water infiltration. Accumulation of excess adsorbed Na (exchangeable Na) on the soil exchange complex (negatively charged sites on soil colloids and organic matter) causes soil colloids and organic matter to disperse, resulting in the destruction of soil structure, particularly the larger pores, and reduced permeability of soil to water and gases. Dispersion of soil organic matter produces a black mucky mat on moist soil surfaces; such soils are referred to as black alkali soil. III­6

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Exchangeable Na on the soil exchange complex is most frequently appraised with the sodium adsorption ratio (SAR) of the soil solution, since the analytical method for determining + 2+ 2+ 0.5 exchangeable sodium is time­consuming. SAR is defined as the ratio (Na )/(Ca +Mg ) when + 2+ 2+ 0.5 units are in millimoles per liter or as (Na )/[(Ca +Mg )/2] when units are in meq/L. Thus, 0.5 0.5 SAR has units of (millimoles/liter) or (meq/L) . SARadj (or adjusted SAR) is sometimes used to account for the tendency of calcium to decrease in the soil solution due to the precipitation of calcite. The theoretical computation of SARadj is based on the Langelier saturation index (Langelier, 1936), which is used widely in the water industry. A much simpler method of calculating SARadj is based on tabular values of expected Ca concentration from a matrix of ratio of HCO3- to Ca and EC of the water 2+ 2+ 2+ (technically referred to as adj RNa by Ayers and Westcot, 1985). This expected Ca concentration 2+ replaces Ca in the denominator of the SAR expression. Ayers and Westcot (1985) calculated SAR and SARadj for 250 water samples from throughout the world and noted that SAR was within ±10% of SARadj for most waters. In waters with more of a tendency to form carbonate minerals, SARadj may be markedly higher than SAR. This may be the case for some but not for all recycled waters with elevated HCO3- concentrations as a result of chemicals used in wastewater treatment processes. Ayers and Westcot (1985) now recommend taking SARadj ´ 0.5 as a more correct representation of SAR adjusted for the effects of calcite precipitation in irrigated soils. The rate of water infiltration into soils and soil hydraulic conductivity are affected by the interaction between the SAR and the EC of the water. Moderate to high SAR (sodicity) may cause soil colloids to disperse and result in reduced infiltration rates. A relatively high EC (salinity or electrolyte concentration) may cause soil colloids to coagulate, resulting in increased infiltration rates. An illustrative interaction of SAR and EC is shown in Figure III.A.2 (after Henderson, 1955). The impact of electrolyte concentration (EC) on hydraulic conductivity of Columbia silt loam is shown by the curve labeled SAR­0. Note that reduced hydraulic conductivity of this soil may be partially overcome by increasing water salinity for waters of lower SARs. Another view of the SAR­EC relationship (Figure III.A.3) is widely used to evaluate the effects of sodicity and salinity on rates of water infiltration in medium­ to fine­textured soils (Ayers and Westcot, 1985). Note that the SAR poses the most hazard to soil permeability at low ECs and that this hazard may be partially overcome by increasing EC. Experience in California water recycling practice indicates that nearly all such recycled waters fall within the safe zone of this graph—that is, no reduction in infiltration rate occurs (B. Sheikh, personal communication). III­7

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A second parameter of sodicity widely used before the advent of SAR is residual sodium carbonate (RSC), which is equal to (HCO3 2- + CO3 2- ) – (Ca + Mg ) in meq/L (Eaton, 1950). A 2+ 2+ positive difference between carbonate alkalinity and hardness would result in residual carbonates. + When combined with Na , these carbonates disperse soil organic matter, forming a black residue known as black alkali on the surface of soils. RSC exceeding 1.25 meq/L may cause increasing problems with sodicity. III.A.4. Other Constituents of Concern Irrigation waters, especially recycled waters, may contain a number of other constituents of concern. They include nitrogen (nitrates and ammonium), bicarbonates, residual chlorine, and constituents that might plug the small orifices of irrigation systems, such as drip irrigation emitters. Nitrogen Some natural waters, especially groundwaters, may contain appreciable concentrations of nitrates from geochemical origins. Other ground­ and surface waters may have an accumulation of nitrates leached from excessive land applications of chemical fertilizers, animal manures and dairy wastewaters, biosolids, and other products of wastewater origin. Untreated municipal wastewaters contain organic nitrogen and some NH3. The organic nitrogen is oxidized in wastewater treatment into NH3 (and NH4 + ) and is further oxidized into NO2- and NO3- . The oxidation of NH4 + by microbes results in the production of protons (H ), and hence the acidified + water is typically neutralized by chemicals such as lime. Thus, Title 22 recycled waters typically contain from 15 to about 50 mg of N of NO3- and NH4 + ions/L, which is equivalent to 41 to 136 lbs. of N per ac­ft of water applied (lbs./ac­ft = mg/L ´ 2.72). These sources of nitrogen, if not taken into account when fertilizing the plants with nitrogen, may cause excessive vegetative growth, lodging, delayed or reduced flower bloom, and the leaching of excess N beyond the root zone, possibly contaminating groundwater. Phosphorus Unlike the discharge of phosphorus­containing effluents into surface water, the land application of phosphorus from recycled waters is of little concern, given its low solubility, use by plants, and lack of mobility in the soil column. On the other hand, phosphorus is sometimes the limiting nutrient for algal productivity in surface water. Therefore, the discharge of III­9

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phosphorus­containing effluents into surface waters may result in proliferic algal growth. Ponds, lakes, and reservoirs holding recycled water may present algal management problems. Carbonates Carbonate ions (HCO3- and CO3 2- ) are not found at elevated concentrations in waters and soil solutions since carbonate minerals have very low solubility product constants. In fact, the precipitation of carbonate minerals during evapoconcentration of soil water reduces the accumulation of soluble salts in soils. However, the deposition of carbonates from overhead sprinklers on fruits, such as table grapes, apples, and pears, and on flowering plants lowers the market quality of the fruits and flowers. The deposition of carbonates may also lead to plugging of irrigation systems. The precipitation of carbonates of calcium and magnesium is of concern in constructed root zones used on golf greens and sports fields in arid climates when they are irrigated with water unusually high in alkalinity (carbonates) and hardness (calcium and magnesium). Such carbonate precipitation may lead to plugging of pores in sands because sands have less surface area than do clays. Consequently, it would be advisable to ensure that carbonate ions are not excessive in the recycled water. Residual Chlorine Molecular chlorine (Cl2) and its related chlorine compounds—sodium hypochlorite (NaOCl), calcium hypochlorite (Ca[OCl]2), and chlorine dioxide (ClO2) —are used, usually as a final step in the treatment process. Hydrolysis of chlorine compounds forms hypochlorous acid (HOCl), which ionizes into hypochlorite ion (OCl- ) (Metcalf and Eddy, 2003). The combined concentration of molecular chlorine, hypochlorous acid, and hypochlorite ion is known as free available chlorine, which is a very good disinfection agent. However, free available chlorine reacts rapidly with ammonia and other organic nitrogen usually present in wastewaters, forming combined available chlorine, which is not as effective as free chlorine in disinfecting water. Unless the ammonia and organic nitrogen in wastewaters have not been oxidized to nitrate by the treatment processes, the primary disinfection agent in chlorinated recycled water will be combined chlorine. Excessive levels of free residual chlorine in recycled waters that have been oxidized to the nitrate form—more than 5 mg/L—may result in root and foliar damage to sprinkler­irrigated plants, since free chlorine is a strong oxidizing agent. However, as pointed out above, most III­10

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Table III.A.1. Plugging potential of irrigation water used in drip irrigation systems (Nakayama, 1982). Degree of potential restrictions on use Type of problem Little Slight to moderate Severe Physical: suspended solids, mg/L < 50 50–100 >100 Chemical: pH <7 7–8 >8 TDS, mg/L <500 500–2,000 >2,000 Manganese, mg/L <0.1 0.1–1.5 >1.5 Iron, mg/L <0.1 0.1–1.5 >1.5 Hydrogen sulfide, mg/L <0.5 0.5–2.0 >2.0 Biological: bacterial population <10,000 10,000–50,000 >50,000 (maximum number/mL) recycled waters contain little if any free residual chlorine, most of which if present will dissipate fairly quickly upon exposure to the atmosphere. Because of stricter trihalomethane (THM) standards, there has been some changeover in the use of chlorine to chloramine for disinfection of potable waters. However, chloramine compounds have been identified as corrosive to certain metals and degrade rubber and some plastic elastomers in earlier irrigation equipment (AWWA Research Foundation, 1993). Most irrigation equipment is now manufactured with components resistant to chloramine degradation, but occasionally an older irrigation system that has been retrofitted to recycled water may demonstrate problems. Fortunately, PVC (polyvinyl chloride) and CPVC (chlorinated polyvinyl chloride) compounds typically used to manufacture irrigation pipe, fittings, and lake liners appear to be resistant to chloramine degradation. Clogging Constituents Recycled waters contain physical, chemical, and biological constituents that might cumulatively clog small orifices in sprinkler irrigation systems, such as drip emitters (Nakayama, 1982). Physical constituents include suspended solids, mainly sand fractions. Chemical constituents include those that form precipitates, such as calcium carbonate, iron and manganese hydroxides, and hydrogen sulfides. Biological constituents may result from microbial activities, such as the production of hydroxides and sulfides from microbially mediated redox reactions. III­11

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Table III.B.1. Water quality classification proposed by Wilcox and Magistad, U.S. Salinity Laboratory in 1943 (Wilcox, 1948). Quality characteristic Class I Class II Class III Excellent to good Good to injurious Injurious to unsatisfactory EC, dS/m <1 1–­3 >3 Boron, mg/L <0.5 0.5–2.0 >2.0 Chloride, mg/L <178 178–355 >355 Sodium, % of cations <60 60–75 >75 Table III.A.1 summarizes the plugging potentials of certain levels of these three types of constituents in water applied in drip irrigation systems (Nakayama, 1982). Constituents in most recycled waters, especially waters receiving Title 22 tertiary treatment, pose little potential restriction on use for virtually all of these parameters. A possible exception is TDS, which may pose slight to moderate potential restriction on use. III.B. Water Quality Guidelines For nearly a century, chemical constituents in water used to irrigate have been known to have some potential effect on soils and crops. A concerted effort to classify waters according to their suitability for irrigating crops and landscape plants has been made in the past 60 years or so. This section summarizes some earlier guidelines regarding water quality and then focuses on current guidelines. III.B.1. Early Water Quality Guidelines The U.S. Salinity Laboratory in 1943 suggested one of the earliest water quality classification schemes for irrigated agriculture (Wilcox, 1948). It involved four quality characteristics and three classes (Table III.B.1), including salinity (EC), specific ion toxicity (boron or chloride), and sodicity (Na%). Since then, H. Chapman of University of California– Riverside, L. D. Doneen of University of California–Davis, F. Eaton of the U.S. Department of Agriculture, H. Dregne and H. J. Maker of New Mexico State University, and J. P. Thorne and W. P. Thorne of Utah State University have advanced several more classification systems (Lunt, 1963). In 1954, the U.S. Salinity Laboratory published Agricultural Handbook No. 60 (Richards, 1954), which became regarded worldwide as the definitive book on diagnosing and improving saline and alkali soils. Included in the handbook was a diagram for classifying irrigation water (Figure III.B.1) with regard to salinity hazard (EC) and sodium hazard (SAR), each with four levels of hazard for a total of 16 classes. III­12

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Low­salinity C1 waters—those with an EC of less than 0.25 dS/m—can be used to irrigate most crops on most soils with little likelihood that soil salinity will pose a problem. Moderate­salinity C2 waters—those with an EC of 0.25 to 0.75 dS/m—can be used for irrigation without special salinity control practices, if a moderate amount of leaching occurs or moderately salt­tolerant plants are grown. High­salinity C3 waters—those with an EC of 0.75 to 2.25 dS/m— can be used to irrigate only plants with good salt tolerance on soils without restricted internal drainage and possibly with special salt management measures required. Very­high­salinity C4 waters—those with an EC of more than 2.25 dS/m—are ordinarily unsuitable for irrigation but may be used to irrigate highly salt­tolerant crops and under such special circumstances as extensive leaching. Handbook 60 contained tables on fruit, vegetable, forage, and field crops with low, moderate, and high salt tolerances. The sodium hazard was evaluated primarily on physical properties of soils as affected by accumulation of exchangeable sodium on the cation exchange sites and secondarily on specific + ion toxicity of Na . The accumulation of exchangeable sodium is related to the SAR, a soil water parameter discussed above. Unlike salinity hazard, the classification of sodium hazards has a negative slope on the SAR­versus­EC matrix. Low­sodium­hazard S1 waters can be used to irrigate almost all soils with little danger of accumulating harmful levels of exchangeable Na but + not when such Na ­sensitive crops as stone fruits and avocados are involved, since such crops + may accumulate injurious concentrations of Na . Medium­sodium­hazard S2 waters may be used for irrigation of coarse­textured or organic soils with good permeability. Irrigating with these waters will present an appreciable hazard in fine­textured soils with high cation exchange capacity, especially under low LFs. If gypsum is present in the soil, the sodium hazard will be 2+ reduced, since Ca dissolved from gypsum will reduce levels of exchangeable Na. Use of high­ sodium­hazard S3 waters for irrigation may result in harmful levels of exchangeable Na in most soils and will require special soil management, such as good drainage, high leaching, and additions of organic matter. Gypsiferous soils may not develop harmful levels of exchangeable Na with this type of water. Chemical amendments may need to be used to lower exchangeable Na. Very­high­sodium­hazard S4 waters are generally unsuitable for irrigation. Though the U.S. Salinity Laboratory system for classifying irrigation water with regard to EC and SAR (Figure III.B.1) was broadly accepted and applied, some noted that the diagonal lines appeared to have the wrong slope for the permeability of fine­ to medium­textured soils. A water with low sodium hazard and low salinity hazard infiltrates slowly over the long term, while a water with low sodium hazard and medium to high salinity infiltrates at an acceptable rate (see, e.g., Figure III.A.2). SAR can cause soil colloids, especially clay minerals such as smectites, to III­14

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disperse, resulting in a poor rate of water intake, while EC coagulates soil colloids, promoting a good rate of water intake, so that the adverse effects of SAR are partially overcome by higher salinity (Figure III.A.3). Currently, Figure III.A.3 is used to appraise the combined EC­SAR effects on the permeability of soil, while Figure III.B.1 is used to assess the hazards of exchangeable sodium on plants and soils. Agricultural Handbook No. 60 also included a system for classifying permissible limits of boron in waters (Table III.B.2) suggested by Scofield (1936). Boron, an essential element for plants at very low concentrations, is injurious to plants at slightly above essential concentrations. Citrus, stone fruits, and beans are particularly sensitive to boron. (Refer to Chapter V for a listing of boron­sensitive plants.) Handbook 60 additionally contained a system for classifying RSC (Table III.B.3) advanced by Eaton (1950). This parameter is used by many water­testing soil and horticultural laboratories. Waters having an RSC of 1.25 to 2.5 may be used to irrigate if Ca­producing amendments, as well as good soil and water management practices, are also used. In the early 1960s, a number of disagreements arose over the use of various classification systems. In 1963, the University of California Water Resources Center convened a workshop to evaluate various systems for classifying agricultural water (Lunt, 1963), where a consensus was reached on recommendations for a new classification system (Table III.B.4). It excluded consideration of the aforementioned SAR­EC relationship for soil permeability (Figure III.A.2). Table III.B.2. Permissible limits of boron for several classes of irrigation waters (after Scofield, 1936). Concn of boron in irrigation water (mg of boron/L) for: Boron class Sensitive crops Semitolerant crops Tolerant crops 1 <0.33 <0.67 <1.00 2 0.33–0.67 0.67–1.33 1.00–2.00 3 0.67–1.00 1.33–2.00 2.00–3.00 4 1.00–1.25 2.00–2.50 3.00–3.75 5 >1.25 >2.50 >3.75 Table III.B.3. Suitability of waters for irrigation based on RSC (after Eaton, 1950). RSC in meq/L Suitability <1.25 Probably safe 1.25–2.5 Marginal quality for irrigation >2.5 Not suitable for irrigation III­15

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In the mid­ to late 1960s, nitrate contamination of groundwaters was increasingly detected. This phenomenon was of concern to public health, as some of those waters served as sources of drinking water. One investigation conducted in the Upper Santa Ana River Basin (Ayers and Branson, 1973) identified excess fertilizers, animal manure, dairy wastewaters, and municipal and industrial wastewaters as the major sources of nitrate in groundwaters. The safe disposal and wise use of nitrogen­containing wastes and their salt content became a concern. The California Regional Water Quality Control Boards requested reevaluations of land treatment and recycling of animal wastes and dairy wastewaters. In 1973, a University of California Committee of Consultants convened to reevaluate and revise water quality guidelines for producing crops (Ayers and Branson, 1975). The revised guidelines (Table III.B.5) were streamlined to categorize certain levels of constituents in irrigation water as presenting either “no problems,” “increasing problems,” or “severe problems.” The electrolyte effect on the permeability of soil, specific ion toxicity differentiated by root absorption versus foliar absorption, the significance of NH4 + and NO3- in waters, and the deposit of carbonates on plants were all considered in these guidelines. Table III.B.4. Recommended water classification system, UC Water Resources Center (Lunt, 1963). Salinity hazard Low Medium High Very high EC, dS/m <0.75 0.75–1.50 1.50–3.00 3 Sodium hazard Low Medium High Very high 0.5 SAR, (mM/L) <3 3­5 5­8 >8 Semitolerant Hazardous Safe for sensitive Sensitive crops Tolerant crops Boron hazard crops will show for nearly crops will show injury will show injury injury all crops Boron, mg/L <0.5 0.5–1.0 1.0–2.0 2.0–4.0 >4.0 Medium tolerant Moderately Safe for sensitive Sensitive crops Chloride hazard crops will show tolerant crops will crops will show injury injury show injury Chloride, mg/L <71 71–142 142–284 >284 RSC hazard Probably low Intermediate Probably high RSC, meq/L <0 0–1.25 >1.25 III­16

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Table III.B.5. Guidelines for the interpretation of quality of water for irrigation (Ayers and Branson, 1975). Problem and related Water quality guidelines constituent in irrigation water Increasing No problem Severe problems problems Salinity, ECw in dS/m <0.75 0.75–3.0 >3.0 Permeability, ECw in dS/m >0.5 <0.5 <0.2 Adjusted SAR <6 6–9 >9 Specific ion toxicity From root absorption: Sodium, adjusted <3 >3 SAR Chloride, mg/L <142 142–355 >355 Boron, mg/L <0.5 0.5–2.0 2.0–10.0 From foliar absorption (sprinklers): Sodium, mg/L <69 >69 Chloride, mg/L <106 >106 Miscellaneous NH4­N + NO3­N, mg/L <5 5–30 >30 HCO3, mg/L (only with <90 90–520 >520 overhead sprinklers) pH normal range: 6.5–8.4 III.B.2. Current Guidelines 1 Ayers and Westcot (1976) developed the most widely used water quality guidelines for irrigation (Table III.B.6). Given in detail in Irrigation and Drainage Paper 29 of the Food and Agriculture Organization (FAO), these guidelines were based on the 1975 1 University of California Committee of Consultants Guidelines with some revisions. The FAO guidelines included recommended maximum concentrations of trace elements in irrigation waters (Table 111.B.7), much of which was based on accumulation of trace elements in soils under long­ term normal irrigation and potential uptake of trace elements by plants (Pratt, 1972). Later, Ayers and Tanji (1981) recommended that the FAO guidelines could also be used for irrigating crops with wastewater. A few years later, Ayers and Westcot (1985) revised the FAO guidelines. These guidelines were further adapted in a guidance manual on irrigation with reclaimed municipal wastewater (Pettygrove and Asano, 1986). Currently, the FAO guidelines are applied internationally in irrigated agriculture and nationally in the use of recycled water to irrigate crops and landscapes. 1 Robert Ayers, a UC Extension Water Specialist and coauthor of the 1975 UC Committee of Consultants’ Water Quality Guidelines (Ayers and Branson, 1975), took a sabbatical leave at the FAO in Rome. There he worked with soil scientist Dennis Westcot of FAO, who currently is a private consultant after service with the California Central Valley Regional Water Quality Control Board, to adapt and expand on the UC Committee of Consultants Water Quality Guidelines. III­17

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degraded, it requires higher management skills to safely use that water. These guidelines should be used as a first approximation for considering the suitability of water for irrigation and then modified for local conditions as needed. Not plant specific, they may be too restrictive for some more tolerant plants and perhaps not restrictive enough for some more sensitive plants. The guidelines are based on the following assumptions: Yield Potential No restrictions on use indicate full production capability without the use of special management practices. Restrictions on use indicate that the choice of crop may be limited or that special management practices are required to attain full production capability. This situation may not be as predominant a concern in the case of landscape plants, as their visual appearance is more important than is harvested yield or biomass. Table III.B.7. Recommended maximum concentrations of trace elements in irrigation waters that might limit crop production due to toxicity and/or limit the utilization of the produce (adapted from National Academy of Sciences and National Academy of Engineering [1972] and Pratt [1972]). Element Recommended maximum concn (mg/L) Al (aluminum) 5.0 As (arsenic) 0.10 Be (beryllium) 0.10 Cd (cadmium) 0.01 Co (cobalt) 0.05 Cr (chromium) 0.10 Cu (copper) 0.20 F (fluoride) 1.0 Fe (iron) 5.0 Li (lithium) 2.5 Mn (manganese) 0.20 Mo (molybdenum) 0.01 Ni (nickel) 0.20 Pb (lead) 5.0 Se (selenium) 0.02 Sn (tin) N/A Ti (titanium) N/A W (tungsten) N/A V (vanadium) 0.10 Zn (zinc) 2.0 III­19

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Site Conditions Soil texture ranges from sandy loam to clay loam with good internal drainage and shallow water table controllable to within 2 m of land surface. The climate is semiarid to arid with low rainfall. Rainfall does not contribute much to meeting crop water demand or to meeting the leaching requirement of crops. The guidelines are too restrictive when rainfall is high during the growing season.. Methods and Timing of Irrigation Normal surface or sprinkler irrigation methods are used. Water is applied when available soil water depletion is less than 50% before the next irrigation. LF, the ratio of root zone drainage to infiltrated irrigation water, is 0.15 or greater. These guidelines are too restrictive for drip irrigation or for daily to frequent irrigations. Water Uptake by Crops The root water extraction pattern is about 40–30–20–10% of crop reference evapotranspiration (ETo) from surface root zone quartile to bottom quartile. Each irrigation event results in leaching of salts in the upper root zone and accumulation of salts in the bottom root zone. The average root zone salinity in soil water (ECsw) is estimated to be about three times greater than in the applied water (ECw), and the average root zone salinity of the soil saturation extract (ECe) is estimated to be about 1.5 times ECw. These relationships are based on a steady­ state LF of 15 to 20% (or 0.15 to 0.20). Restriction on Use The three categories of restrictions on use, which are somewhat arbitrary due to the lack of a clear­cut specific boundary and the gradual occurrence of changes, are based on studies, observations, and experiences in the field. A change of 10 to 20% above or below a numeric guidance value may have little significance for crop yield if other guidance values have no restrictions or less restriction. Moreover, the management skill of the water user could alter the degree of restrictions. For instance, an ECw of 0.85 dS/m may not necessarily pose a restriction on use if the LF exceeds 15%, because there will be only a small accumulation of salts in the root zone. However, if the water SAR is 9, there may be slight to moderate problems in water intake 2+ rates that might be corrected with water or soil amendments containing Ca . Moreover, + sprinkler­applied water with a SAR of 9 could severely damage Na ­sensitive plants, such as stone fruits. III­20

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The FAO guidelines, though accepted worldwide in irrigated agriculture and widely used since 1976, should be considered part of an initial effort to evaluate the suitability of waters for landscape irrigation. As landscape professionals gain experience in the use of recycled waters to irrigate landscape plants, they may need to consider additional constraints or modifications in addition to potential plugging (Table III.A.1) and RSC (Table III.B.3). III.C. Quality of Recycled Waters Generally speaking, if a source water is of acceptable quality to irrigate landscape plants, the recycled water will likewise be of acceptable quality for irrigating landscape plants, since recycled water usually accrues small amounts of dissolved minerals—from 150 to 400 mg of TDS/L (Asano et al., 1985). There are, however, some exceptions: First, as discussed above, if a significant number of water softeners utilizing sodium chloride are used in the community served by the wastewater treatment plant, there may be significant accumulation of Na and Cl- ions in the recycled water. This accumulation might pose + a specific ion toxicity hazard to sensitive plants, as well as adversely affect soil water infiltration, especially when highly trafficked turf is being irrigated. Second, recycled waters often contain 15 to 40 mg of nitrogen/L as organic­N, NH4­N, and NO3­N (Asano et al., 1985). Since each milligram of N per L equals 2.72 lbs. of N per ac­ft of water, this source of N needs to be taken into account when considering the plant’s need for N. Third, recycled waters are neutralized with bases such as lime or soda ash, because the oxidation of NH4­N to NO3­N produces acidity, substantially raising the RSC of the water. Fourth, recycled waters may contain sufficiently high concentrations of boron to injure boron­sensitive plants. Fifth, some recycled waters may contain constituents that tend to plug parts of sprinkler irrigation systems, such as the small orifices in drip emitters. III.C.1. Representative Composition of Recycled Waters Data were compiled on the composition of Title 22 recycled waters used to irrigate landscapes and agricultural crops at four different sites (Table III.C.1) in three different studies (Sheikh et al., 1990; Shaw et al., 1995; and West Basin Municipal Water District, 2004). The data reported are mean or median values for samples of water obtained at regular intervals: residual chlorine and turbidity were monitored continuously, the pH and EC were monitored daily, SS (suspended solids) were monitored weekly, dissolved minerals were monitored monthly, and III­21

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floodplain representative of many San Diego County golf courses. Recycled water reported for the West Basin Water Recycling Facility irrigated landscapes at Toyota Motor Sales USA, the Home Depot National Training Center, Goodyear, golf courses such as Victoria Golf Course in Carson, and other landscaped sites. The recycled water reported for Monterey County in a Monterey Wastewater Reclamation Study for Agriculture at Castroville was a demonstration study of recycled water used to irrigate crops of artichokes, broccoli, cauliflower, lettuce, and celery. It is also representative of recycled waters used on golf courses in Monterey County. As pointed out above, the amount of dissolved minerals in recycled water primarily depends on the quality of the source water, plus the small increase of additional salts, as well as nutrients, from water usage. III.C.2. Evaluation of Representative Recycled Waters Using FAO Guidelines The FAO Water Quality Guidelines (Ayers and Westcot, 1985), though based on work with irrigated agricultural crops, are generally applicable to landscape plants (Ayers and Tanji, 1981). However, the FAO guidelines for sodicity may not be applicable for all landscape plants. That is, sodicity is more of an issue in landscape than in agricultural lands since landscape plants, such as turf, are permanent, eliminating the tillage conducted between the growing seasons of most irrigated crops to improve soil tilth and control weeds. Nevertheless, the four representative recycled waters discussed above could be evaluated by using the FAO guidelines and their three classifications regarding the extent of restrictions on the use of waters: no restrictions, slight to moderate restrictions, and severe restrictions (Table III.C.2). The recycled water from the West Basin Water Recycling Facility in Los Angeles County falls into two categories of severe restriction on use: excess residual chlorine and excess N. It is expected that the 5.6 mg of residual chlorine/L in this recycled water will tend to be dissipated within the distribution system and to be rapidly dissipated at the time of its application. The excess N is more problematic if normal fertilization rates are practiced with this water. The 32.4 mg of N/Lin this recycled water contains about 88 lbs. of N/ac­ft. About 352 lbs. of N would be applied for a seasonal water application rate of 4 ac­ft/acre, a level of application that fulfills the N requirement of most plants. Therefore, commercial N fertilizer would not be needed when one is irrigating with this recycled water. Furthermore, some N is expected to be leached beyond the root zone because plants do not consume 100% of the applied N as either applied fertilizer or as N dissolved in water. Slight to moderate restrictions on use are expected for each of the four recycled waters discussed above, depending on the species of plants, the types of soil, and the practice of water III­23

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reduce permeability for some types of soils, especially those with silt loam texture and that are dominated by smectite clay minerals, which swell upon wetting and shrink upon drying. Third, the recycled water from the West Basin Water Recycling Facility has an RSC of 1.7 mg/L and may disperse soil organic matter and reduce water infiltration, as noted at the Victoria Golf Course in Carson, Calif., where acids are being injected into the water line to ameliorate the problem. Fourth, all four waters are judged to have slight to moderate restrictions on use because of the Na and Cl- specific ion hazard when irrigated by both sprinkler and surface irrigation + methods. A solution to this problem is that plants sensitive to Na and Cl- could be replaced by + more tolerant ones. The recycled waters discussed above (Monterey Wastewater Reclamation Study for Agriculture, Torrey Pines, and Whispering Palms) have from 9 to 14 mg of total N/L and thus may have slight to moderate restrictions on use. Landscape plants are generally more sensitive to boron than are agricultural crop plants, and the numerical limits presented in Table III.B.5 (Ayers and Branson, 1975) may be more appropriate: i.e., no restrictions, less than 0.5 mg of B/L; slight to moderate restrictions, 0.5 to 2.0 mg/L; and severe restrictions, 2 to 10 mg/L. If the Ayers and Branson (1975) limits are applied, water produced by the West Basin Water Recycling Facility will fall in the slight to moderate restriction in use for landscape irrigation. In summary, certain characteristics of recycled waters require attention when the waters + are used to irrigate landscape plants. Such characteristics include total N, EC, SAR, RSC, B, Na , and Cl- ions. Therefore, wastewater reclamation facilities should ideally be designed and operated so that these constituents in the water produced do not pose a hazard or restrictions on use on landscape irrigation. However, for reclamation facilities that have multiple customers with various end users, it may not be cost effective to install multiple plant modifications to meet specific user requirements. III.D. A Case Study: Use of Potable and Recycled Waters on Turfgrasses at Whispering Palms Site Of some concern is the chemical composition of the leachates that result from deep percolation of irrigation drainage into groundwater basins and the load of salts and nitrates that leach from the root zone of landscapes irrigated with recycled waters, especially when the underlying groundwater serves as a source of drinking water. Observed data on the quality of root zone drainage in landscape irrigation are unfortunately scarce. Leachate composition is difficult to predict because of the numerous processes that affect the composition and amount (load) of leachates produced. These processes include the following: III­25

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· loss of pure water to the atmosphere from evapotranspiration (ETo), with salts remaining in the soil solution · leaching from irrigation and rainfall · net mineral precipitation from evapoconcentration and mineral dissolution from the chemical weathering of soil minerals · uptake by plants of such ions as NH4 + , NO3- , Na , K , Cl- , etc. + + · cation exchange between solution cations and exchangeable cations on the soil exchange complex, e.g., solution Na replacing exchangeable Ca · transformations such as oxidation of NH4®NO2®NO3, mineralization of organic N®NH4, and denitrification of NO3®NO2®N2O®N2 · adsorption of solution ions onto surfaces of iron and aluminum oxides of soil clays, e.g., boron. A simplified approach for estimating the leaching of salinity from root zones into the vadose zone, the unsaturated zone above the water table of groundwater basins, is presented in Chapter IV. A case study on observed leaching of salt and nitrates comparing potable and recycled water irrigation on turfgrasses follows. Shaw et al. (1995) conducted extensive studies in San Diego County of the use of recycled waters to irrigate landscapes. Of particular interest is the field trial using potable and recycled water from the Whispering Palm facility to irrigate plots of turfgrass—an ideal site in that many San Diego County golf courses are similarly located in river basin floodplains and that the drainage of these floodplain soils allows leaching to eliminate the accumulation of salts due to irrigation with moderately saline waters. The soils at this site are identified as Grangeville fine sandy loam and Tujunga sand. As noted in Table III.D.1, the recycled water used in this study contained concentrations of constituents greater than those found in the associated potable water. Chemical analyses for both waters range quite widely due to changes in blending of imported waters: Colorado River through the Colorado River Aqueduct and northern California water from the California Aqueduct were blended with local surface and well waters in San Diego County. The EC, SAR, and NO3­N of the recycled water are of particular concern from a water quality perspective (Shaw et al., 1995). III­26

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Table III.D.1. The mean and range (parentheses) of water quality of potable and recycled waters used in the Whispering Palms turfgrass study (Shaw et al., 1995). Property measured Potable water Recycled water pH (6.8–8.0) (6.8–7.7) EC, dS/m 0.98 (0.77–1.09) 1.41 (1.24–1.63) TDS, mg/L 630 (493–685) 900 (736–1043) 0.5 SAR, mM/L 2.7 (2.2–3.1) 4.8 (4.0–5.5) Na, mg/L 105 (85–116) 185 (147–212) Cl, mg/L 117 (75–190) 198 (82–269) Boron, mg/L 0.15 (0.08–0.23) 0.50 (0.28–0.67) NO3­N, mg/L 0.19 (0.05–0.51) 11.2 (2.5–23.7) NH4­N, mg/L 0.07 (0.04–0.26) 0.18 (0.01–0.66) Table III.D.2. Irrigation data for turfgrass from Whispering Palms study (Shaw et al., 1995). Property measured Cool­season turf Warm­season turf CIMIS ETo, in. 90.7 90.7 Calcd. ETc, in. 74.5 54.1 Rainfall, in. 25.1 25.1 Potable Recycled Potable Recycled Irrigation, in. 103.4 106.7 78.3 88.7 Total applied water, in. 128.5 131.8 103.4 113.8 Drainage, in. 54 57.3 49.3 59.7 LF, 0.42 0.42 0.48 0.52 Drainage/total applied water The turfgrasses selected for this study included two warm­season varieties, common bermudagrass (Cynodon dactylon) and kikuyugrass (Pennisetum clandestinum), and two cool­ season varieties, tall fescue (Festuca arundinacea) and Kentucky bluegrass (Poa pratensis)/perennial ryegrass (Lolium moltiflorum) mixture, all grasses commonly planted in San Diego County golf courses, parks, and landscapes. Kikuyugrass, not a commonly recommended turfgrass, has become the dominant species in many parks and golf courses due to its adaptability and invasiveness. Warm­season turfgrasses have a higher tolerance of drought and salinity, requiring about 25% less water than do cool­season turfgrasses and achieving peak growth and appearance during the summer. These grasses become dormant in the winter, unlike cool­season grasses, which grow most and are at their aesthetic best in the fall through spring. Consequently, warm­season grasses are being used more often and are overseeded with ryegrass while dormant in the winter to maintain an acceptable appearance. The experimental design at Whispering Palms involved plots of cool­season and warm­ season turfgrasses, randomly assigned, with each having three replicates of irrigation water (potable and recycled waters) and three replicates of turfgrass species. Each plot was 20 ft by 20 III­27

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ft for water treatment, with each subdivided into two subplots that were 20 ft by 10 ft for cool­ and warm­season grasses, for a total of 12 plots with 24 subplots. Irrigation scheduling Table III.D.3. Soil saturation extract analyses within and below the turfgrass root zone in Whispering Palms study (Shaw et al., 1995). Within root zone Below root zone 0–24­in. depths 24–36­in. depths ECe SAR NO3­N ECe SAR NO3­N Grass 0.5 0.5 dS/m mM/L Mg/L dS/m mM/L Mg/L species LF Irrigation type Bermudagrass 0.48 Potable water 3.3 5.8 0.2 2.5 3.8 2.1 Bermudagrass 0.52 Recycled water 3.0 5.6 1.0 2.2 5.0 3.1 Kikuyugrass 0.48 Potable water 2.7 8.2 0.8 1.7 5.6 3.2 Kikuyugrass 0.52 Recycled water 3.7 10.4 0.4 1.9 6.5 2.2 was conducted by the water budget method and involved real­time data about local weather from the California Irrigation Management Information System (CIMIS) weather station. The duration of irrigation was proportionally adjusted to the changes in reference ETo. Turfgrass ETo (ETc) in inches per day was assumed as approximately equal to 0.6 ´ ETo for warm­season grasses and 0.8 ´ ETo for cool­season grasses. Table III.D.2 presents the irrigation data. The cool­season grasses received an average of 105 in. of irrigation, and warm­season grasses received an average of 84 in. The total water applied, irrigation plus rainfall, for cool­season grasses averaged 130 in. and for warm­season grasses averaged 109 in. from January 1993 through November 1994. The calculated turf ETo for cool­season grasses was 74.5 in. and for warm­season grasses was 54.1 in. The difference between total applied water and ETo is the drainage out of the root zone, which averaged 56 in. for cool­season grasses and 54 in. for warm­season grasses. The LF, the ratio of drainage to total applied water, averaged 0.42 for cool­season grasses and 0.50 for warm­season grasses. Shaw et al. (1995) determined dissolved mineral constituents within the root zone and below the root zone of the turfgrasses. This database, along with the irrigation data, may be used to estimate mass emission of nitrates and salts from the root zone into the vadose zone, the unsaturated zone above the groundwater table. Table III.D.3 presents soil data on EC, SAR, and NO3­N determined in the extracts of saturated soil pastes (a standard method of analyzing soil samples [Richards, 1954]). The results show that, with a moderate LF of about 0.5, the EC of extracts of soil paste in both recycled and potable water was comparatively low within the root zone and below, even though the recycled water had an EC of 1.4 dS/m. These soil salinity values are within the acceptable limits of salt tolerance for these turfgrasses. The FAO guidelines indicate that a water of 1.4 dS/m may have slight to moderate restriction on use. The guideline is based on an LF of 0.15 to 0.20 and is not plant specific. With an LF higher than 0.2, the III­28

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restriction on use may be lessened. At Whispering Palms, the LFs ranged from 0.4 to 0.5, except during the rainy season, when LFs were about 0.7. The results also show that NO3­N within the root zone and below for both potable and recycled water treatments was low, although the recycled water had an NO3­N concentration of 11.2 mg/L. This finding indicates that turfgrasses are heavy feeders of N and can effectively recover N from fertilizer and irrigation water. Clearly, N in the recycled water should be considered for meeting part of the N requirement of grasses. Shaw et al. (1995) believe that excessive nitrate­N leaching losses can be avoided by adjusting fertilizer N applications and lowering the LF. The FAO guidelines suggest that Whispering Palms recycled water would have slight to moderate constraints in use based on the total N content of the water. Table III.D.4. Estimated mass loading of nitrates and salts from the root zone into the vadose zone at the Whispering Palms study (after Shaw et al., 1995). Loading value Bermudagrass Bermudagrass Kikuyugrass Kikuyugrass Potable Recycled water Potable water Recycled water water Applied N fertilizer 544 544 544 544 lbs./acre N content in water 4 225 4 225 lbs./acre Total N applied 548 769 548 769 lbs./acre NO3­N in drainage 2.1 3.1 3.2 2.2 mg/L LF 0.48 0.52 0.48 0.52 N in drainage 47 84 72 59 lbs./acre N drained 8.6 10.9 13.1 7.7 % TDS in applied water 630 900 630 900 mg/L TDS in applied water 5.59 9.05 5.86 9.32 tons/acre TDS in drainage 1,412 1,062 1,180 1,294 mg/L TDS in drainage 8.78 7.19 7.33 8.76 tons/acre a TDS drained 150 77 125 94 % a Initial soil salinity (ECe) was 1.7 dS/m compared to 1.1 to 1.2 dS/m for the others. III­29

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The SARs for the plots of kikuyugrass irrigated with both potable and recycled waters were slightly greater than the SARs for the plots of bermudagrass. Shaw et al. (1995) had some concerns about loss of permeability and reduction in infiltration rates, but these were not observed during the 23 months of the study. The sustainability of soil permeability over a longer period has not been ascertained. Contrary to the FAO guidelines, the SAR and EC of the recycled water in this study did not result in slight restrictions on use. If Figure III.A.3 is used as a guide, the Whispering Palms recycled water falls within the no­reduction­in­permeability category. It should be noted that heavy foot and mower traffic on golf courses with fine­textured (clayey) soils sometimes leads to problems with water penetration. Table III.D.4 presents the estimated mass loadings of nitrates and salts into the vadose zone based on volume of drainage past the root zone (Table III.D.2) and concentrations of nitrates and salts found below the root zone (Table III.D.3). With regard to nitrates, the total N load applied to the grasses was 548 lbs. per acre for the potable water application and 769 lbs. per acre for the recycled water application. Applied N fertilizer was a major source, along with N in the recycled water. The N load discharged with the percolating water below the root zone was only 8 to 13% of the total N applied. This finding confirms that turfgrasses are heavy feeders of N. It should be noted that clippings from mowed grasses remain on site and contribute to organic matter in the soil, a small portion of which becomes bioavailable upon mineralization. In contrast, the mass emission of salts differed from nitrates. The load of salts applied with irrigation water ranged from 5.6 to 9.3 tons per acre. The salt concentration below the root zone ranged from 1,060 to 1,410 mg/L or a discharge load ranging from 7.2 to 8.8 tons/acre. Unlike N, most of the applied salts—77 to 125%— were leached out of the root zone. The calculated deviations from 100% are considered acceptable for salt leaching due to the complex chemical reactivity of salts. The bermudagrass irrigated with potable water was an exception with a 150% salt leaching. A plausible reason for this is that the initial soil salinity in the plots of bermudagrass irrigated with potable water had an ECe of 1.4 dS/m, while other plots had an initial soil ECe of 1.1 to 1.2 dS/m. Shaw et al. (1995) monitored the aesthetic quality of the turfgrasses throughout the study, using a standard turf­scoring procedure that involves a scale from 1, which equals dead turf, to 9, which equals perfect color, texture, density, absence of pests, and overall quality. Both the cool­ season and warm­season turfgrasses scored an average of about 6.5, indicating acceptable quality. Turfgrasses are known to be relatively tolerant of Na , Cl- , and B but are not well + documented in the literature. They are mowed at heights ranging from 1.5 to 3.0 in. for cool­ season grasses and less than 1.0 in. for warm­season grasses. For both cool­season and warm­ + season turfgrasses, the concentrations of Na in the soil extracts ranged from about 115 to 345 III­30

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mg/L and the concentrations of Cl- ranged from about 106 to 320 mg/L. Concentrations of B ranged from about 0.25 to 0.6 mg/L in the extract of saturated soil pastes. Salts accumulating in the leaf tips are removed during mowing, and consequently leaf tip burns from salts, Na , Cl- , or + B are not typically observed. However, there is some concern that leaving mulched mowed clippings on site can cause Na , Cl- , and B to return to the soil, resulting in long­term increases of + these constituents in the soil. The FAO guidelines for these specific ions indicate slight to moderate restrictions on use of the recycled water for Na and Cl- and none for B. + This study demonstrated that recycled water can be beneficially used to irrigate turfgrasses, thus conserving potable waters. Relatively few problems were observed in this 23­ month study. Shaw et al. (1995) further state that reliability of the quality of recycled waters is important. Any significant changes should be reported to the user of recycled water or to a professional landscape advisor, so that appropriate agronomic and water management options can be taken to avoid problems. Variations in the quality of recycled water, the types of soil, management practices, patterns of use, climate and expected quality of turfgrass should be noted when evaluating the benefits of using recycled water to irrigate golf courses. This case study demonstrated that the FAO Water Quality Guidelines are a useful guide that is perhaps somewhat conservative for the irrigation of turfgrasses. Nitrate leaching losses at this study’s site were kept to a minimum, while salts were extensively leached out. III.E. References Asano, T., R. G. Smith, and G. Tchobanoglous. 1985. Municipal wastewater: treatment and reclaimed characteristics, p. 2­1–2­24. In G. S. Pettygrove and T. Asano (ed.), Irrigation with Reclaimed Municipal Wastewater: a Guidance Manual. Lewis Publishers. AWWA Research Foundation. 1993. Chloramine effects on distribution system materials. Project no. 508). AWWA. Ayers, R. S., and R. L. Branson (ed.). 1973. Nitrates in the Upper Santa Ana River basin in relation to groundwater pollution. Agric. Experiment Station Bull. 861:1–60. Ayers, R. S., and R. L. Branson. 1975. Water quality. Guidelines for interpretation of water quality for agriculture. University of California Cooperative Extension report. University of California. Ayers, R. S., and D. W. Westcot. 1976. Water quality for agriculture. FAO irrigation and drainage paper 29. Food and Agriculture Organization of the United Nations, Rome, Italy. Ayers, R. S., and D. W. Westcot. 1985. Water quality for agriculture. FAO irrigation and drainage paper 29, rev. 1. Food and Agriculture Organization of the United Nations, Rome, Italy. III­31

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Chapter IV. Salinity Control in the Root Zone and Deep Percolation of Salts from the Root Zone into Groundwater Basins K. Tanji, S. Grattan, and B. Sheikh IV.A. Root Zone Salinity IV.A.1. LF and LR—Root Zone as a Whole IV.A.2. LF and Profile Salt Distribution IV.A.3. Soil Salinity as Affected by Irrigation and Rainfall IV.A.4. Reclamation Leaching IV.A.5. Pore Volume Reclamation Leaching IV.B. More Complex Treatment of Soil Salinity IV.B.1. Chemical Reactivity of Salts in Water in the Soil IV.B.2. Equilibrium Chemical Model—WATSUIT IV.B.3. Application of WATSUIT to Quartile Root Zone Salt Accumulation IV.B.4. Application of Reactive Mixing Cell Model for Salt Accumulation and Leaching IV.C. Concerns about Salt Loading into Regional Groundwater Basins IV.D. Summary IV.E. References Since soil salinity reduces the availability of soil water to plants due to osmotic effects (Chapter III), salts in the root zone should be kept below the maximum level tolerated by plants for optimal plant performance (Chapter V). When salinity in the root zone exceeds this level, plants experience osmotic stress and their growth is adversely affected. In low­rainfall regions, irrigating plants causes soluble salts to accumulate in the root zone, as most of the salts in the irrigation water remain in the soil after more or less pure water is lost to the atmosphere via evaporation and transpiration (Chapter V). Excessive levels of salinity do not accumulate in the root zone if sufficient leaching occurs: i.e., if rainfall and/or irrigation exceeds the water holding capacity in the root zone and if soil water drains past the root zone, carrying salts with it. Irrigation and drainage water management can play a significant role in keeping soil salinity below the maximum level tolerated by plants. In this chapter, the principles of water and soil salinity management, as well as their practical application, are covered. Included in the appendices are a number of Excel­based hydrosalinity models that can be used to perform some of the calculations that follow. A specific model will be used when appropriate. IV­1

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IV.A. Root Zone Salinity IV.A.1. LF and LR—Root Zone as a Whole Leaching fraction (LF) is defined as the steady state (long­term) ratio of the depth of drainage water (Ddw) that drains past the root zone to the depth of irrigation water (Diw) that infiltrates the root zone. The ratio of Ddw to Diw was formerly defined as the leaching requirement (LR) by the U.S. Salinity Laboratory (Richards, 1954) but presently is known as LF (LR will be defined later), D - D iw et D LF = = dw (IV­1) D iw D iw In the absence of rainfall, the depth of drainage water is the difference between Diw and Det, depth of evapotranspiration (ET). Det, which is equivalent to ETc as defined in Chapter VIII, is defined in this chapter by Det = Deto* Kc (IV­2) where Deto is the depth of reference ET or ETo (obtained, e.g., from a nearby CIMIS weather station) and Kc is the crop coefficient. The FAO Water Quality Guidelines (Ayers and Westcot, 1985) assumed an LF of 15 to 20%, and if Det can be estimated with Equation IV­2, the amount of water needed for irrigation, Diw, can be obtained by rearranging Equation IV­1, D et D w = i (IV­3) ( - LF ) 1 In some quarters, distribution uniformity of water application (DU) is considered in Equation IV­3 as D et D = iw (IV­4) (1 - LF ) * DU IV­2

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The DU with a well­managed irrigation system ranges from 0.70 to 0.95 (Tanji and Hanson, 1990). Correcting for DU will lead to large values of Diw if the irrigation systems are not maintained and operated to achieve high DU, resulting in excessive leaching in much of the irrigated land and root pathogens, such as Phytophtera. In some quarters, the DU is ignored in estimating depth of irrigation. Though a small portion of the irrigated land may be underirrigated while ignoring the DU and plant performance may be adversely affected in that portion, ignoring the DU is acceptable with respect to water conservation in irrigated agriculture. If, however, a high, uniform quality of appearance is desired, as in turfgrass and lawns, then the DU may be considered. Illustrative example IV­1 What is the annual depth of irrigation required for a cool­season turfgrass (such as Kentucky bluegrass) with annual ETo of 50.6 in. in Los Angeles and annual crop coefficient Kc of 0.80, while assuming an LF of 0.20? Det = Deto * Kc = 50.6 in. * 0.80 = 40.5 in. D 40 5 inches . D = et = iw = 50 6 inches . 1 - LF 1 - 0 20 . Handbook No. 60 (Richards, 1954) points out that salinity is inversely proportional to water in its LR expression, i.e., D dw EC iw LF = = (IV­5) D iw EC dw Thus, ECdw is EC iw EC dw = (IV­6) LF The ET process evapoconcentrates soil water as more or less pure water is lost to the atmosphere through transpiration and evaporation, resulting in an increased concentration of salt in the root zone. IV­3

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Illustrative example IV­2 What is the EC of the water draining past the root zone if the irrigation water has an EC of 1.5 dS/m and if LF is 0.20? EC iw 1. dS / m 5 EC dw = = = 7 5 dS / m . LF 0 20 . The steady­state salinity in the drainage water (ECdw) is 7.5 dS/m, and considering the root zone as a whole, the EC of the soil water (ECsw) in the root zone is somewhere between ECiw in the soil surface and ECdw in the bottom of the root zone. If a simple average between these two is assumed, the average root zone salinity is 4.5 dS/m [(1.5 +7.5 dS/m)/2)]. This EC of 4.5 dS/m represents the average root zone ECsw at field capacity (FC) soil moisture. FC refers to the soil water status after adequate irrigation and when drainage stops or reduces to a low rate. Since salt tolerance threshold values for plants are expressed as ECe, the EC of the extract from a saturated soil paste, ECsw, needs to be converted to ECe. The conversion factor used is 0.5, assuming that soil saturation percentage (SP) is twice the soil water content at FC for most soil types. Thus, the average root zone ECe for this example is 2.25 dS/m (4.5 dS/m * 0.5) for an LF of 0.2 and ECiw of 1.5 dS/m. LR is a plant­specific parameter. It is a prescribed value of leaching, so that root zone salinity does not exceed the threshold salinity tolerance of the plant in question. This plant­ specific LR is defined as EC iw LR = (IV­7) 5 * EC a - EC iw where ECa is the plant­specific threshold soil salinity, above which yields decrease in the case of crops and above which performance is reduced in the case of landscape plants, and the factor “5” is an empirically derived factor to account for distribution of salts by soil depth (Rhoades, 1974). Threshold salinity values for plants are reported in Chapter V. IV­4

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IV.A.2. LF and Profile Salt Distribution The distribution of salts in the root zone as affected by root water extraction patterns, Diw, Det, LFs, and ECiw, will now be examined. Figure IV.A.1 divides the root zone into quartiles (four layers) and assumes that most crop plants extract soil water to meet their seasonal ET with a 40– 30–20–10% water extraction pattern (Wq) in the root zone quartiles (Ayers and Westcot, 1985). Based on the previously outlined ET evapoconcentration approach, the LF, the resulting EC at the bottom of each quartile, and the distribution of salts in the root zone can be estimated. Illustrative example IV­4 What is the long­term salt distribution in the root zone of a cool­season grass uniformly sprinkler irrigated with 52.4 in. of water/year (Diw) at proper intervals to meet the water needs of the grass, with water of 1.5 dS/m (ECiw) and ET of 41.9 in./year (Det), assuming that the root water extraction pattern is 40–30–20–10% of ET (Wq)? Use Hydrosalinity Model 1 in the appendices. Table IV.A.1 gives the illustrative computation. First, the LF of each root zone quartile (LFq) is calculated with Eq. IV­10 (given in Table IV.A.1), and then the EC of the soil water leaving the root zone quartile (ECq) is calculated with Eq. IV­6 (given in Table IV.A.1). As water is extracted in the root zone quartile, the LF decreases and the EC of the water draining from the quartile increases. The overall root zone LF is 0.20, and ECdw is 7.50 dS/m. Within the root zone quartile, the EC of the soil water (ECsw) is assumed to be the average between water entering the quartile and water leaving the quartile; e.g., ECsw in the fourth quartile is the average between 5.34 (EC3) and 7.50 dS/m (EC4) or 6.42 dS/m. The average root zone ECsw for the soil profile as a whole is 3.99 dS/m [(1.5+2.21+3.41+5.34+7.5 dS/m)/5]. Figure IV.A.2 is a plot of the calculated values in Table IV.A.1 as the ECsw­1 curve with a root water extraction pattern of 40–30–20–10% of crop ET in the root zone quartiles. This cool­ season grass is assumed to have a rooting depth of 12 in., and thus each quartile is of 3­in. increments. Curve ECsw­2 is for the same case, except the extraction pattern is 60–25–10–5% of ET. Because the extraction in the first root zone quartile is higher (60 versus 40%), the ECsw in the bottom of the quartile is slightly higher in ECsw­2 than in ECsw­1. But the ECdw (bottom of the fourth quartile) is the same because the overall LF is the same. IV­6

74.
Figure IV.A.3 contains a plot of salt profiles in which only the LF varies for a cool­ season grass that has an annual ET of 41.9 in., is irrigated with water EC of 1.5 dS/m, and has a root water extraction pattern of 60–25–10–5% of ET. This plot clearly shows that root zone salinity is highly regulated by the LF, assuming there is no impediment to root zone drainage. The threshold salinity of plants is given as average root zone salinity in the extract of saturated soil paste or ECe in dS/m. The data plotted in Figure IV.A.3 are in terms of ECsw, and the conversion to ECe is made by multiplying ECsw by 0.5. The average root zone ECsw values for LFs of 0.05, 0.10, 0.20, and 0.30 are 10.62, 6.64, 4.25, and 3.27 dS/m, respectively, which, when converted to average ECe, would be 5.32, 3.34, 2.12, and 1.64 dS/m, respectively. If the threshold salinity of a cool­season grass is assumed to be ECe of 3 dS/m, then using an ECiw of 1.5 dS/m will require an LF slightly greater than 0.10—an LF of 0.11 to be exact (see Equation IV­8). Salinity distribution with LF varied 35 30 25 ECsw, dS/m LF=0.05 20 LF=0.10 15 LF=0.20 LF=0.30 10 5 0 0 2 4 6 8 10 12 14 Soil depth, inches Figure IV.A.3. Salinity profile for a cool­season grass irrigated with ECiw of 1.5 dS/m, ET of 41.9 in./year, and root water extraction pattern of 60–25–10–5% of ET. IV.A.3. Soil Salinity as Affected by Irrigation and Rainfall Up to now, the effects of rainfall on root zone salinity and leaching were ignored. The equations on water and salts extended for annual rainfall include one from Richards, 1954: ( D + D ) - D D LF = iw rw et = dw (IV­11) iw + rw D + D D + D iw rw iw rw IV­8

75.
where Drw denotes depth of annual effective rainfall (infiltrated into the soil). The effective annual rainfall is about 50% of annual rainfall for a typical amount of rainfall, ground cover, and topographical slope on irrigated lands of California. In ( D * EC ) + ( D * EC ) ECiw rw = iw iw rw rw (IV­12) + ( D + D ) iw rw ECrw refers to EC of rainwater and ECiw+rw is the volume­weighted average EC of the mixed supply water. These same equations may apply for a second source of supply water other than rainwater, such as a blend of two irrigation waters of differing water salinities. A more precise treatment can be realized if monthly to weekly values are used instead of seasonal data in equations IV­11 and IV­12. Illustrative example IV­5 What is the long­term distribution of salts in the root zone of a cool­season grass, assuming the same conditions as in Example IV­4 but with an effective annual rainfall of 8 in. and an EC of rainwater of 0.01 dS/m? Use Hydrosalinity Model 2 in the appendices. Table IV.A.4 shows that computations similar to those used in Table IV.A.1 are employed to solve this problem. The EC of mixed supply water is ( D * EC ) + ( D * EC ) ( . * 52 4 + ( . * 8 0 EC + = iw iw rw rw = 1 5 . ) 0 1 . ) = 1 31 dS / m . iw rw ( D + D ) 52 4 + 8 0 . . iw rw Figure IV.A.4 plots the salt accumulation pattern computed in Table IV.A.2 and is compared to that calculated in Table IV.A.1. The average root zone salinity (ECsw) is 4.0 dS/m with irrigation water and 2.68 dS/m with irrigation water plus rainfall. The salt accumulation is less, as rainwater EC is very low. The LF is greater (0.31 versus 0.20), since 8.0 in. of effective rainwater is added to the 52.4 in. of irrigation. Therefore, effective rainfall should be taken into account when it is a significant fraction of the infiltrated water and salt accumulation is of concern. In California, since most rainfall occurs during the winter and most irrigation occurs in the summer, the aforementioned calculations may differ slightly from actual calculations. Winter rains will leach the salts accumulated in the previous summer and fall months. These rains may serve as reclamation leaching, if there is sufficient rainfall. IV­9

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states that the concentration of salt in a particular depth of soil and at a particular time is the average of the concentration of salt in that particular depth from a previous time (resident salt) and the concentration of salt entering from a depth above at that particular time (invading salt). Illustrative example IV­6 Given an initial soil salinity in 6­in. depth increments of a 48­in. clay loam soil profile with ECsw of 10, 12, 18, 12, 6, 4, 4, and 4 dS/m, calculate the degree of reclamation leaching with water having an ECiw of 1.5 dS/m. Figure IV.A.5 gives the results of using Equation IV­15 with an initial salinity given as curve j­1. If the soil texture is clay loam, it would have a field capacity of about 4 in. of water/ft of soil or in this case 2 in. per 6­in. soil depth. This 2­in. depth of water is the increment of reclamation leaching water applied. After leaching with one increment of ECiw of 1.5 dS/m, the salt profile is given by curve “j,” two increments of leaching water by curve “j+1,” and so forth. The salt bulge in the 18­in. soil depth is slowly displaced downward with each increment of reclamation leaching. The average ECsw for the initial salt profile for each increment of reclamation leaching is 8.75 (j­1), 8.35 (j), 7.82 (j+1), 7.14 (j+2), 6.37 (j+3), 5.57 (j+4), 4.80 (j+5), and 4.10 dS/m (j+6). This model is applicable to leaching by rainwater or any other ECiw water. Use Hydrosalinity Model 3 in the appendices. Reclamation salt leaching 20 ECsw, dS/m j­1 15 j 10 j+1 j+2 5 j+3 J+4 0 j+5 0 20 40 60 j+6 Soil depth, inches Figure IV.A.5. Results of reclamation leaching for initial salt profile given by curve j­1 with ECiw of 1.5 dS/m. The salt bulge at the 18­in. soil depth is displaced downward with each leaching water increment j, equivalent to a 2­ in. depth of reclamation water. IV­11

78.
IV.A.5. Pore Volume Reclamation Leaching Reclamation leaching of constructed root zones established on modern sports fields and golf greens with soil mixes and sand do not often behave in a manner similar to that of native soils. The constructed root zones often incorporate high­sand­content soil mixes of prescribed depths, typically 12 to 14 in., layered profiles designed to perch the water table, and/or confined root zones with impermeable barriers that use a combination of subsurface irrigation/drainage systems. Moreover, common turfgrass management practices such as light and frequent sand topdressing result in an accumulation of high­sand­content root zones over a native soil base lacking subsurface drainage. The low­cation­exchange­capacity, high­sand­content root zone mixtures result in a more rapid development of salt accumulation and stress symptoms than is found in native clay and loam soil profiles and therefore require more frequent leaching events. Thus, adaptation and modification of leaching protocols become necessary. Carrow et al. (2000) utilize a leaching method based upon pore volume (PV), the total pore space (Rhoades and Loveday, 1990), in the constructed root zone. Table IV.A.1 presents the estimated reclamation needs based on soil texture. Table IV.A.1. Estimated reclamation leaching needs based on soil texture. No. of in. of water Soil texture PV in % per 12­in. soil to fill to PV Sand (>95% sand content) 35 4.2 Loamy sand 38 4.56 Sandy loam 42 5.05 Clays 50 6 PV equivalent of water required to Soil texture leach 70% of total a salts Sand (>95% sand content) 0.7 b Sandy loam 1.00–1.25 b Loams 1.50–2.50 b Clays 2.50–4.00 a PV equivalent values are adjusted by Carrow, Huck, and Duncan based on experience. b Use higher values for 2:1 lattice shrink swell cracking clays and lower values for 1:1 noncracking clays. IV­12

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Illustrative example IV­7 What is the depth of water needed to leach salts on a high­sand (>95% sand) golf course or sports field to a depth of 16 in. overlying a subsurface drainage tile lines? This example was contributed by Mike Huck (personal communication). A sandy soil texture has a PV of 35%, requiring a 4.20­in. depth of water per 12­in. soil depth to fill its PV. Thus, for a 16­in. reconstructed soil depth, 5.60 in. of water (4.20 in. of water ´ 16 in./12 in.) would be required. For these high­sand greens, a PV equivalent of 0.70 is used to achieve approximately 70% leaching of soluble salts. Therefore, 3.90 in. of water (5.60 in. of water ´ 0.70) applied would leach 70% of the salts across the 16­ in. soil depth. IV.B. More Complex Treatment of Soil Salinity In previous sections of this chapter, it has been assumed that salinity in the water is a conservative parameter; i.e., it does not chemically react. Strictly speaking, dissolved mineral salts in waters are chemically reactive. For example, they participate in mineral dissolution, precipitation reactions, and cation exchange reactions. This section describes some common water chemical processes that affect salinity and the use of chemical equilibrium computer models to appraise more quantitatively the chemical reactivity of waters. IV.B.1. Chemical Reactivity of Salts in Waters in the Soil The major chemical reactions occurring in the soil that affect salinity are the dissolution and precipitation of such minerals as calcite (CaCO3) and gypsum (CaSO4∙2H2O) and the exchange of cations—Na, Ca, Mg, NH4, and K—between those soluble forms in the soil solution and those forms adsorbed onto the soil exchange complex, which consists of negatively charged clay minerals and soil organic matter. Figure IV.B.1 depicts this chemistry of soil solutions (Tanji, 1990). Note that interactive chemical reactions involve particular chemical ion species. For example, the dissolution of gypsum (CaSO4∙2H2O) produces free calcium ions (Ca ) and free sulfate ions (SO4 2− ). The free 2+ Ca ion may replace exchangeable magnesium ion on the soil exchange complex, and SO4 ions may form the neutrally charged MgSO4 ion pair and monovalently charged NaSO4 − ion pair. As cation exchange and ion pair reactions occur, they raise the solubility of gypsum to a level higher than that of the solubility of gypsum in distilled water. Because of this complexity and the interactive nature of such soil chemical reactions, chemical equilibrium models are used to evaluate chemical speciation and the equilibrium chemistry of waters and soil solutions. IV­13

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Figure IV.B.1. Interactive chemical reactions in soil water systems (Tanji, 1990). The solution phase consists of + − completely dissociated ions such as Na as well as incompletely dissociated ion (ion pairs) such as NaSO4 . IV.B.2. Equilibrium Chemical Models—WATSUIT The Water Suitability Determination Model (WATSUIT) by Rhoades and Dell’Osso (1976) will be used first to evaluate equilibrium water chemistry in this section and then to evaluate root zone salt accumulation in the next section. To illustrate the complex nature of interactive chemistry in soils, changes in water chemistry resulting from the evapoconcentration of Colorado River water will be examined, using WATSUIT. Evapoconcentration of waters in this computer model is evaluated by applying the LF; i.e., an LF of 1.0 will have no evaporation, while an LF of 0.1 means the original volume of water has been evapoconcentrated 10­fold. As a first approximation, the concentration of salt is increased 10­fold. Table IV.B.1 presents the results of evapoconcentrating Colorado River water from 1­ to 20­fold. The column titled “River water” gives the initial concentration in milliequivalents per liter (meq/L) (WATSUIT uses combining chemical concentration units or meq/L), while concentration in milligrams per liter (mg/L) is given in parentheses. The data for an LF of 1.0 are the computed chemical equilibrium concentration with no evapoconcentration. Note that Colorado River water is saturated with respect to calcite (CaCO3) and that this mineral precipitates out of the water under equilibrium IV­14

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“reactive” is predicted by WATSUIT, which considers reactive water chemistry. Reduced salt accumulation in the reactive case is caused by the deposition of calcite and gypsum as irrigation water is increasingly evapoconcentrated by root water extraction. The concentrations of calcite forming in the root zone quartiles are plotted in meq/L, a range of values equivalent to about 100 to 600 ppm (mg/L = meq/L ´ mg/meq = meq/L ´ 50.04 mg/meq). The LF for the first to fourth quartiles in this case is 0.68, 0.44, 0.28, and 0.20. The average ECsw for the nonreactive case is 3.38 dS/m and for the reactive case is 2.34 dS/m. If one accounts for the natural chemistry of the water and appropriate chemical reactions, the effective salinity in this Colorado River water is reduced by about 30%. Figure IV.B.3 plots salt accumulation from the use of Colorado River water at LFs that range from 0.05 to 0.40 (5 to 40% of infiltrated water) as calculated by WATSUIT. The average ECsw values for LFs of 0.4, 0.3, 0.2, 0.1 and 0.05 are 1.77, 2.03, 2.34, 3.38, and 4.66 dS/m, respectively. If the threshold salinity of the plants is known, the LFs can be managed to keep soil salinity at a tolerable level during the use of Colorado River water. Comparison of rootzone salinity with reactive and nonreactive models ECsw, dS/m, and CaCO3, 14 12 10 meq/L nonreactive 8 watsuit 6 meq/LCaC03 4 2 0 0 2 4 6 Root zone quartiles Figure IV.B.2. Comparison of root zone salt accumulation from irrigation with Colorado River water at an LF of 0.2, assuming on the one hand that salts in the water behave conservatively (nonreactive) and on the other hand that they are chemically reactive as predicted by WATSUIT. Also plotted is the concentration of calcite precipitating as its solubility product constant is exceeded. The deposition of calcite reduces salt accumulation in the root zone quartiles. IV­16

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Root zone salt accumulation with LF varied 14 12 10 LF 0.4 ECsw , d S/m LF 0.3 8 LF 0.2 6 LF 0.1 4 LF 0.05 2 0 0 1 2 3 4 5 Root zone quartiles Figure IV.B.3. Salt accumulation from irrigation with Colorado River water at various LFs as predicted by WATSUIT. IV.B.3. Application of Reactive Mixing Cell Model for Salt Accumulation and Leaching The reclamation leaching mixing cell model (IV.A.4) can be extended to consider more flexible and dynamic vadose zone conditions (Tanji, 2000) by EC q j = 0 5 * ( q j -1 + ( EC , . EC , / LF )) (IV­16) q - 1 j , q In this model, there is no constraint on the number of increments of soil depth to be considered (such as quartile root zone), and each increment will be of specified length (inches or feet). ECq,j−1 is the initial concentration of soil salinity in each soil depth, and accumulated salt may be nonuniformly distributed in the soil profile. The evapoconcentrating effect of root water extraction in each depth increment is considered by LFq. If there is no root water extraction at that soil depth, LFq assumes a value of unity (1.00) and salt transport can be calculated below the rooting depth. IV­17

84.
Traditional root­water extraction patterns are typically given in root zone quartiles (four depths) in the rooting soil depth. They could be subdivided further into any number of depths, such as eight depth increments instead of four. When water infiltrates the soil surface, the ECq−1,j in Equation IV­16 is the salinity of the applied water of a given source or the salinity of multiple sources of water, which is the volume­weighted average ECiw, as in Equation IV­12. This model is applicable to reclamation leaching, too, should one assume LFq is unity and reduce Equation IV­16 to IV­15. Equation IV­16 may be further extended to consider the chemical reactivity of waters. Given the mineral precipitation data of Colorado River water in Table IV.B.1, the reduction in accumulated salts and EC as a function of LF can be estimated. Table IV.B.2 contains calcite and gypsum precipitation data from Table IV.B.1. The precipitation of these minerals will reduce the resulting EC upon evapoconcentration of Colorado River water. For instance, the EC of the water subjected to an LF of 0.1, assuming no chemical reactions, is 10.3 dS/m. The estimated reduction in EC from mineral precipitation is 3.67 dS/m for an LF of 0.1, and the resulting EC after chemical reaction is 6.63 dS/m. The reduction in ECs is quite substantial at smaller LFs. Figure IV.B.4 plots the reduction in EC in Colorado River water due to mineral precipitation at various LFs. A curve fitting this plot yields ECsw = -0.0265/LF2, which is then inserted into Equation IV­16 to account for chemical reactivity of Colorado River water as EC - 1 j q , 0 0265 . ECq j = 0 5 * ( q j -1 + [( , . EC , ) - ( )] (IV­17) LF q LF 2 Table IV.B.2. Mineral precipitation with evapoconcentration of Colorado River water and estimates of reduction of accumulated salt concentration. The ECiw of water is 1.03 dS/m. LF = LF = 1.0 LF = 0.4 LF = 0.3 LF = 0.1 0.05 Calcite precipitation, mg/L 97 122 280 1,490 3,256 (row 1) Gypsum precipitation, mg/L 1,207 4,340 (row 2) Sum of precipitation, mg/L 97 122 280 2,697 7,596 (row 3 = 1 + 2) Estimated EC reduction from precipitation, dS/m 0.132 0.166 0.381 3.669 10.335 a (row 4 = 3/735) Nonreactive evapoconcentration, dS/m 1.03 2.575 3.433 10.3 20.6 (row 5 = ECiw/LF) Reactive evapoconcentration, 0.898 2.409 3.052 6.631 10.265 dS/m (row 6 = 5−4) a Reader should assume that 735 mg/L equals 1 dS/m. IV­18

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Illustrative example IV­8 What is the degree of salt accumulation in a 30­in. soil profile in which a cool­season grass is grown with a water EC of 1.5 dS/m? The crop ET is 41.9 in., and total irrigation is 52.4 in., resulting in an LF of 0.20. The rooting depth is 21 in. with root water extraction pattern of 30–30– 15–10–5–5–5% of ET. The initial soil salinity values in 3­in. depth increments are 1.5, 2.5, 3.0, 3.5, 4.6, 5.9, 7.4, 9.7, 8.7, and 5.9 dS/m. Use Hydrosalinity Model 4 in the appendices. Figure IV.B.5 contains the initial soil salinity (ECsw) given by the curve labeled “0 in.” The hydrosalinity model calculates salt distribution after each 5.24­in. irrigation in this particular problem. After 10.5 in. of irrigation, the salt bulge below the root zone has been displaced. By 21 in. of irrigation, the salt profile is approaching the final salt profile that one finds after 52.4 in. of irrigation. IV.C. Concerns about Salt Loading into Regional Groundwater Basins The Southern California Salinity Coalition is concerned about regional salt balance in California’s south coastal region. There is also a Water Replenishment District that is concerned with salt loading into its groundwater basins. This admittedly complex problem will be addressed in this section only relative to the use of recycled water for irrigating landscapes. Water from irrigation and rainfall not used by plants that infiltrates past the root zone are known as deep percolation. This deep percolation carries salt into the groundwater basin. Such transport of salt into a groundwater basin may or may not be of crucial significance. It is a matter of concern where irrigation occurs above an unconfined aquifer with a relatively shallow water table. This phenomenon is less the case where salt from a farm or from landscaped fields deeply percolates into a salt sink that is used on a limited basis or where the aquifer is confined below a deep aquiclude and there is little seepage across this barrier. Whatever the site­specific circumstances, it is necessary to address the ability of salt leaching below the root zone to eventually accumulate in the subsoil or in the aquifer underlying it. The length of time it takes for the accumulation of salt in groundwater basins to reach serious levels depends on a number of factors, including natural and artificial recharge to the aquifer, amount of rainfall, extraction of water with wells, interflow to adjoining aquifers, and the effects of such local hydrogeophysical characteristics as vertical faults. Consequently, deep percolation of salts into regional groundwater basins cannot be straightforwardly determined to always or never be a problem. Under certain conditions, however, it can become a significant problem if not mitigated. IV­20

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Depending on the flow path and depth to the water table in an unconfined aquifer, deep percolation through the vadose zone may take from months to decades to reach the surface of the saturated zone. The extent of deep percolation may be estimated as the difference between infiltrated water and ET. Deep percolation is usually estimated as the LF, a decimal fraction of the ratio of deep percolation and infiltrated water, where deep percolation is the difference between infiltrated water and ET losses. The amount of deep percolation in landscape irrigation may vary widely. Intensively irrigated turf and lawns with shallow rooting systems may have an LF that ranges from 0.4 to 0.6. Less intensively irrigated landscape covered by deep­rooted trees and shrubs may have an LF ranging from 0.1 to 0.4. When recycled water is used instead of potable water to irrigate a landscape, the LFs are expected to be about the same or slightly higher. A case study presented in Chapter III compared the extents of nitrate and salt leaching for potable water irrigation and recycled water irrigation of turfgrasses. Assuming that salts are nonreactive in the root zone, under steady­state conditions, the mass of salts present in deep percolation from the root zone would be the same as that introduced by irrigation and rainfall. Due to the concentrating effects of ET, however, the concentration of salt in deep percolation is greater. The degree of evapoconcentration may be approximated from the product of salt concentration in the applied water and the reciprocal of LF, i.e., 1/LF (Tanji, 2002). For instance, if the EC of the water (ECw) is 1 dS/m, the EC of deep percolation from the root zone for an LF of 0.6 is 1.7 dS/m (1 dS/m ´ 1/0.6), for an LF of 0.4 is 2.5 dS/m, and for an LF of 0.2 is 5 dS/m (see previous sections of this chapter for further details on soil salinity). If the salts are also assumed to be nonreactive in the vadose zone beneath the root zone, the EC of the water reaching the water table would remain the same as that of the root zone deep percolation. But typically, there is a net accrual of dissolved mineral salts in deeply percolating water through the vadose zone and rarely a net deposit of salts (Tanji et al., 1967). Salt mass, which is the product of salt concentration and volume of water, may be obtained by converting EC in dS/m to total dissolved solids (TDS) in mg/L and surface depth of water into volume per unit area irrigated. For instance, salt concentration is obtained by a factor of 634 mg of TDS/L per dS of EC/m (sometimes a factor of 735 is used), so that an EC of 1 dS/m contains 634 mg/L. The concentration of salts in deep percolation per acre­foot (ac­ft) of water (Cdp) will be the product of salt concentration (634 mg of TDS/L) and the factor 0.00126 ton of salt per ac­ft of water per mg of TDS/L or 634 ´ 0.00126 or 0.8 ton of salt per ac­ft. The surface depth of deep percolation (Ddp) is obtained from the product of LF and infiltrated irrigation water (Diw). If seasonal Diw is assumed to be 5 ft and LF to be 0.4, the Ddp is 0.4 ´ 5 ft or 2 ft and the volume of deep percolation per unit area (Vdp) irrigated will be 2 ac­ft/acre. Finally, the mass IV­21

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loading of salts in deep percolation (Mdp) is the product of salt concentration (Cdp) and volume per unit area (Vdp). For this example, if Cdp is 0.8 tons per ac­ft and Vdp is 2 ac­ft/ac, the seasonal Mdp is 0.8 ton salt per ac­ft 2 ac­ft/ac of irrigated land or 1.6 tons per acre. When landscapes previously irrigated with potable waters are irrigated with recycled waters, the concentration and mass of salt in deep percolation may be slightly higher because of the residual accrual of dissolved mineral salts in recycled water. The concentration of such salts in domestic recycled waters is typically 150 to 400 mg/L higher than in potable water for an EC increase of about 0.2 to 0.6 dS/m (Asano et al., 1985). Hence, the mass loading of salts into the groundwater basin from irrigating with recycled water is slightly greater than the mass loading of salts from irrigation with potable waters. And salt loading into the groundwater basin would increase even more if previously unirrigated land is irrigated. The increase in mass loading is based on the assumption that the salts in irrigation water do not react with the minerals in the strata through which they percolate. Assuming that all of the salts arriving in irrigation water will ultimately reach the unconfined groundwater aquifer and that complete blending with the aquifer waters occurs, it is possible to compute the equilibrium concentration increase for salts (or any specific constituent) after decades of time during which irrigation with recycled water continues at a constant rate. An Excel­based program was developed for an unconfined aquifer in Santa Clara Valley (B. Sheikh, personal communication), in which characteristics of the aquifer, rainfall, and a range of irrigation acreages were entered to determine the ultimate impact on groundwater quality. This program, for the most conservative scenario, calculates the acreages that can be irrigated for each given recycled water quality without adverse impact. The quality of deep percolation water from the root zone may, in fact, differ from that of the applied water if salts are reactive, for instance, due to net mineral precipitation or net mineral dissolution. Applied water is evapoconcentrated in the soil solution as more or less pure water is lost to the atmosphere during ET and the salts in the water remain in the soil solution, where they concentrate during the drying phase of irrigation. The solubility product constants of sparingly soluble salts may be exceeded, and mineral precipitation may take place. The minerals that predominantly precipitate in irrigated soils are calcium carbonate (CaCO3) and gypsum (CaSO4∙2H2O). Other minerals that might precipitate include magnesium carbonate, calcium phosphate minerals, and silicate minerals. During the wetting phase of irrigation, the more soluble soil minerals present, such as gypsum and calcic feldspars, may dissolve. Reducing the LF generally reduces the mass of salts in deep percolation because of mineral precipitation (Rhoades et al., 1974). Within the flow path of deep percolation into the vadose and saturated zones, the IV­22

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quality of the groundwater may be subject to change, depending on the chemistry of the substratum materials (Tanji et al., 1967). Assessment of mass loading of reactive salts will require a geochemical model coupled to a transport simulation model (Tanji, 2002). In summary, the mass loading of salts into the groundwater basin from irrigating landscapes with recycled water is expected to be somewhat different from that when irrigating with potable water. Depending on whether salts are reactive in the flow path due to mineral precipitation (a salt sink) or mineral dissolution (a salt source), the mass loading of salts could be less when mineral precipitation dominates and greater when mineral dissolution dominates. Thus, it would be imprudent to predict the combined net effect of salt loading and precipitation or dissolution on the groundwater for all situations without precise site­specific knowledge of geochemistry and recycled water characteristics. Currently, the Santa Clara Valley Water District, in collaboration with the California Department of Water Resources and the University of California–Davis, is conducting a field study to determine the actual impacts of irrigation with recycled water on soil water chemistry as leachate moves beyond the root zone toward an unconfined water table (Ashktorab, 2005). The results of that study are expected to become available in 2007. IV.D. Summary Salts tend to build up in the root zone of actively transpiring plants because more or less pure water is lost to the atmosphere through evaporation and transpiration while dissolved mineral salts in the irrigation water remain in the soil solution. One principal means of controlling root zone salinity is by LF, which is defined as the ratio of water draining past the root zone and the applied water. For most waters and most plants, an LF of 0.15 to 0.20 is more than adequate to keep soil salinity at less than harmful levels (Ayers and Westcot, 1985). This chapter covered the principles and applications of LF. The FAO approach of computing salt accumulation in quartile root zone by considering the root­water extraction pattern, the LF, and the EC of applied water was covered in detail. The impacts of rainfall on salt leaching and/or mixed­quality supply waters as well as reclamation salt leaching were also considered. Simple Excel­based hydrosalinity models were used to demonstrate these concepts and practices. These models assume salinity to be a conservative parameter, i.e., not a chemically reactive parameter. This assumption on salinity may be appropriate for conditions of high LFs (i.e., >0.3) and/or for waters that do not tend to form calcite and gypsum upon evapoconcentration. IV­23

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Appendix: Hydrosalinity Model 3Hydrosalinity Model 3 for Illustrative Example IV-5Example IV-5:A clay loam soil profile is salt affected and requires reclamation leaching. TheECiw of the water available for reclamation is 1.5 dS/m, and the EC of soil water (ECsw) in thesoil profile in 6-in. depth increments is 10, 12, 18, 12, 6, 4, 4, and 4 dS/m. What will be thesalinity in this profile if about 1 ft. of reclamation leaching is applied?Computational model: Eq. IV-15 (mixing cell transport model) ECq,j = 0.5 * (ECq,j-1 + EC q-1,,j) EC = EC of soil water q = Specified space (soil depth) increment in inches j = Time increment or leaching event number Eq. IV-15 states that the salt concentration in a particular soil depth q and a particular time j is the average of salt concentration from a previous time (resident salt) and salt concentration entering from a soil depth above (q-1) at that particular time (invading salt). Calculated EC is ECsw or EC at field capacity soil moisture.Soil texture Field capacity In. of water per ft of soilSand 1.2Loamy sand 1.9 Assume that this clay loam soil has a fieldSandy loam 2.5 capacity of 4 in. of water per ft of soil orLoamy sand 3.2 2 in. of water per 6 in. of soil.Silt loam 3.6Sandy clay loam 3.5 The depth of water applied will be in 2-in.Sandy clay 3.4 increments for this case.Clay loam 3.8Silty clay loam 4.3Silty clay 4.8Clay 4.8Initial valuesSoil depth ECsw (dS/m)0–6 in. 10 NOTE: Any changes made in the input data will automatically6–12 in. 12 cause new values to be calculated in the computations below and12–18 in. 18 the plot.18–24 in. 1224–30 in. 630–36 in. 436–42 in. 442–48 in. 4ECiw 1.5 dS/m HM3-1

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Chapter V. Tolerance by Landscape Plants of Salinity and of Specific Ions C. Grieve, L. Wu, L. Rollins, and A. Harivandi V.A. General Information Regarding Salt Tolerance V.A.1. Defining Plant Salt Tolerance V.A.2.Response of a Plant to Salinity V.A.3.Symptoms of Salt­Related Stress V.B. Salt Tolerance of Trees, Shrubs, and Ground Covers V.B.1.Findings from Recent Research V.B.2.Other Sources of Information V.C. Salt Tolerance of Floricultural Plants V.D. Salt Tolerance of Turfgrasses V.E. Salt Tolerance of Native Plants V.F. Sensitivity of Plants to Specific Ions V.F.1.Sensitivity of Trees, Shrubs, Ground Covers, and Floricultural Plants V.F.2.Sensitivity of Turfgrasses V.G. Effects of Environment and Management V.H. Gallery V.I. References In many communities where recycled water is available, the salinity of the recycled water is somewhat higher than the salinity of municipal drinking water. Therefore, in using recycled water to irrigate golf courses, parks, and other landscapes, it may be beneficial to include salt­ tolerant plants, as much as possible, in a landscape’s design. The information in this chapter is provided in the hope that it will help park designers, landscapers, maintenance personnel, and others who work with plants to specify, install, and nurture trees, shrubs, ground covers, floricultural plants, and turfgrasses that can thrive when irrigated with recycled water. Quite a few landscape plants can withstand small or moderate amounts of salt; many are listed in this chapter. Because native Californian plants are favored for park design by the cities of Los Angeles and San Diego and by a number of other communities and individuals in the state, we have included salt tolerance information for native plants to the extent that it is available. V­1

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The responses of plants to salts are manifested in two ways. The osmotic effect produced by total salinity decreases the soil water potential, which causes water in the soil to become less available to plants. And when specific constituents (ions) of salts are present in high concentrations, they can disrupt the plant’s mineral nutrient status, sometimes becoming toxic. At + − times, concentrations of ions such as sodium (Na ), chloride (Cl ), and boron (B) in soil or irrigation water, or both, can prove to be a major constraint in choosing plants or in deciding where to position plants within a landscape. We describe some of the effects of these salt ions on plants and the concentrations at which the ions can become a problem. In addition, we outline a number of management practices that can be used to minimize salt injury to plants. When one is preparing for landscape irrigation with recycled water, environmental quality is an important consideration, especially when the landscape is situated within an urban area. To use the lists of plants in this chapter successfully, information regarding water quality, irrigation management, physical and chemical properties of the soil, and any unfavorable environmental conditions should be obtained and thoroughly reviewed. In addition to choosing plant species that are sufficiently salt tolerant, the landscape professional must select species that adapt well to local climates. California has many different climatic zones ranging from cool, relatively dry, temperate regions in the inland valleys and high mountains to extremely dry, hot deserts to humid, foggy zones along the coast. Since information on the adaptation of plants to climate is readily available elsewhere, we will not further cover the topic in this chapter. V.A. General Information Regarding Salt Tolerance V.A.1. Defining Plant Salt Tolerance The salt tolerance of a plant is often defined as the plant’s inherent ability to withstand the effects of high salts in the root zone or on its leaves without significant adverse effects. The actual salt tolerance of a plant will vary, depending on the growth stage at which salinization is initiated and the final level of salinity to which the plant is subjected (Lunin et al., 1963). Another reason for variation is that the genes that determine a plant’s salt tolerance function in combination with other genes, some of which influence both quantitative traits and environmentally influenced traits, such as salt tolerance (Shannon, 1997). A crop’s salt tolerance can be described as a complex function of its yield decline in response to salinity. The yield response curve is typically valid for a range of concentrations of salts and is sigmoidal in shape. Mathematical descriptions of these relationships have proven useful for crop simulation modeling (van Genuchten and Hoffman, 1984). However, because crop V­2

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survival rates tend to be very low at high salinities, the validity of the bottom part of the yield response curve is often in doubt. Maas and Hoffman (1977) proposed a two­piece linear model described by two parameters: the threshold (electrical conductivity of the extract of a saturated soil paste [ECe] at which significant yield reduction begins), and the slope (percentage of expected yield decline per unit increase in salinity above the threshold value). In landscape plants, aesthetic quality of the plants is more important than yield of crop plants. Nevertheless, the concept of salt tolerance is of value for landscape plants. V.A.2. Response of a Plant to Salinity Lauchli and Epstein (1990) conclude that salinity is stressful for many plants because of two concurrent processes: the osmotic effect and specific­ion effects described earlier. The authors examine the various mechanisms by which plants respond to osmotic effects and to the effects of specific ions. They point out that a plant typically responds to the osmotic effects of salinity by absorbing salt from the medium and by synthesizing organic solutes internally so as to make the water potential gradient more favorable for water uptake. To evaluate what is known about the responses of plants to salinity, Lauchli and Epstein review and then summarize results from a number of studies on the topic. They describe how plants respond during the two successive stages of growth—development and vegetative growth. They conclude the following: · It is not possible to establish a distinct dividing line between saline stress, on the one hand, and lack of stress, on the other. Instead, a continuum exists between the two. · The sensitivity of a plant to salinity changes during the development of the plant. · The integration of responses in the whole plant is critical for the health and survival of a plant under saline conditions. · Highly salt­tolerant plants (halophytes) tend to absorb salt ions from the medium and sequester them in the vacuoles of cells. Such plants also manufacture organic solutes to balance the osmotic changes that occur in the cell cytoplasm. · Salt­sensitive plants, referred to as nonhalophytes or glycophytes, tend to exclude sodium and chloride from their shoots and, especially, from their leaves. Consequently, when subjected to salinity, glycophytes must rely more extensively on the synthesis of organic solutes than do halophytes. · The presence of calcium at elevated concentrations sometimes can help to mitigate the adverse effects of salinity. V­3

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The initial and primary effect of salinity, especially at low to moderate concentrations of salt, results from osmotic effects (Munns and Termaat, 1986). Maturity may be delayed or advanced, depending on the species. For example, salt­related stress in wheat accelerates its development and causes early maturity, whereas salt­related stress in rice causes the plants to mature more slowly. The magnitude of a plant’s response to salinity depends not only on the species but also on the interactive effects of environmental factors such as relative humidity, temperature, radiation, and air pollution (Shannon et al., 1994). Depending on the composition of the irrigation water, ion toxicities or nutritional deficiencies may also arise. These result from a preponderance of a certain specific ion or from competitive effects among cations or anions (Grattan and Grieve, 1999). The osmotic effects of salinity contribute to a reduced rate of growth and to changes in the color of leaves. They also can lead to morphological changes such as smaller leaves or shorter stature or, frequently, to fewer leaves and nodes. Ionic effects generally manifest as damaged leaves or formative plant tissue or as symptoms typical of nutritional disorders. Thus, high concentrations of sodium or chloride ions may accumulate in leaves or in portions of leaves and result in the “scorch” or “firing” of leaves, whereas symptoms of nutritional deficiency are often similar to those that occur in the absence of salinity. Environmental stresses can cause physiological and morphological disruptions in root tissues. Salinity, for example, decreases the integrity and increases the permeability of cell membranes and ultimately results in reduced growth and yield. Such changes may also increase a plant’s susceptibility to invasion by pathogens. Chrysanthemum, a relatively salt­tolerant floral species, showed a definite predisposition to infection by Phytophthora cryptogea when it was affected by salinity. MacDonald (1982) reported a strong positive relationship between the degree of salt stress and the severity of this root rot. V.A.3 Symptoms of Salt­Related Stress The typical observable symptom of a plant injured by salt­related stress is leaf chlorosis (a scorched­like appearance). It is detrimental physically and aesthetically to plants. If subjected to severe salt­related stress, the whole leaf blade may become chlorotic and die. Under moderate salt­related stress, symptoms are similar among salt­sensitive species of plants, although the symptoms on the leaves have a slightly different pattern of distribution. Species assessed to be “highly tolerant” are unlikely to develop any symptoms of salt­ related stress when irrigated with recycled water, even during the dry and warm summer season. Such species include the tree known as Mexican pinon pine (Pinus cembriodes), the shrub known V­4

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as oleander (Nerium oleander), the ground cover red apple iceplant (Aptenia cordifornia), and the grass known as alkali sacaton (Sporobolus airoides). All of these species can tolerate salt spray containing over 1,000 mg of sodium chloride/L, and all are tolerant of soil with a salinity of 10 decisiemens/m (dS/m), or even greater. These plants require only routine management practices. Plants assessed to be “tolerant” are generally able to tolerate spray with water (i.e., wetted foliage from sprinkler irrigation) that contains concentrations of salt equivalent to those found in most recycled waters and generally do not develop apparent symptoms of salt­related stress if the salinity of the soil remains below an ECe of 6 dS/m. However, when the foliage of a tolerant plant is exposed to concentrations of salt exceeding 200 mg of sodium/L and 300 mg of chloride/L, symptoms of salt­related stress begin to appear. Species determined to be “moderately tolerant” can tolerate spray with water containing the concentrations of salts found in most recycled waters. Under such conditions, their aesthetic quality generally remains acceptable, though they may develop symptoms of salt­related stress near the end of the growing season, by which time leaves may have accumulated considerable salt or the salinity of the soil may have exceeded the permissible level. In areas where wet seasons recur cyclically and frequently, moderately tolerant plants will likely do very well through most of the year, even if irrigation is discontinued during the wet seasons. Plants deemed “sensitive” may develop symptoms of salt­related stress under a spray of water containing a concentration of sodium that reaches or exceeds 200 mg/L and a concentration of chloride that reaches or exceeds 400 mg/L, especially if the weather is warm and dry. One such species is liquidambar (Liquidambar styraciflua). Typical symptoms of salt­related and boron­ related stresses for plant species are shown in plates 1 and 2 (Gallery), respectively. Plants sensitive to salt spray from sprinkler irrigation tend also to be sensitive to salinity in the soil. For example, roses may develop severe symptoms of salt­related stress if the salinity in the soil reaches or exceeds 3 dS/m. Research with agronomic plants (Benes et al., 1996) has shown that, for some crops, postwashing (finishing an irrigation, then giving a brief, freshwater rinse) can greatly reduce foliar injury from sprinkling. V.B. Salt Tolerance of Trees, Shrubs, and Ground Covers V.B.1. Findings from Recent Research Based on a recent series of experiments, Wu and Dodge (2005) compiled salt tolerance information for over 200 species of trees and palms, shrubs, and ground covers. Reproduced here as Tables V.B.1.1, V.B.1.2, and V.B.1.3, the lists work fairly well as a plant selection guide for decision­makers in the field of landscape management. V­5

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These lists were developed by a team of University of California–Davis researchers who used sprinkler and drip irrigation systems and waters with salinities near the upper level found in most recycled waters. The field trials were aimed at differentiating the salt tolerance of landscape plants based on the aesthetic effects of salinity, rather than yield reduction as would be done with agronomic crops (Wu et al., 2001). The response of the plants to saline stress was evaluated visually or measured by using image analysis technology (Lumis et al., 1973; Wu et al., 2001; Wu and Guo, 2005). The researchers reviewed the relatively scant literature to date on the relationship between the tolerance by plants of salinity in the water applied to leaves, as compared to tolerance of salinity in the water applied to roots. In one study, these two characteristics were found to have evolved independently between different ecotypes for a species of creeping bentgrass, Agrostis stolonifera L., in a seacoast environment (Ashraf et al., 1986). In another study that involved salt­tolerant creeping fescue cultivars (Festuca rubra L.), the characteristics of leaf wettability were found to be responsible for tolerance of salt spray (Humphreys, 1986). There appears to exist a positive relationship between the salt tolerance by many landscape plants for saline spray and their tolerance of salinity in the root zone (Wu et al., 2001). In some cases, the tolerance for salts entering the plant via its roots was found to be three to four times higher than the tolerance for salts entering the plant through leaves (Wu et al., 2001). Exceptions were certain fruit trees grafted onto rootstocks of different species. Their tolerance of salt spray and tolerance of soil salinity may be unrelated. Based on the results of their field trials, which were conducted in the summer months, and information found in the literature, the researchers estimated the salt tolerances of over 200 species of plants for landscapes (Tables V.B.1.1, V.B.1.2, and V.B.1.3). Although five or six descriptors have been used to categorize the salt tolerance of crop species (Maas and Grattan, 1999), that number was deemed unnecessarily high for differentiating salt tolerance in landscape plants because landscapes often include plants with a wide range of salt tolerance. Instead, these researchers categorized plants using four descriptors for the plants’ ability to tolerate salts in irrigation water: highly tolerant, tolerant, moderately tolerant, or sensitive. They concluded that ranking based on the visual quality of the plants was a practical approach. V­6

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a Data in the table adapted from Wu and Dodge, 2005 (in press). b Tolerances of salt spray are defined by the degree of salt stress symptoms developed in the leaves of the plants and the salt concentrations in the irrigation water as follows: Highly tolerant: No apparent salt stress symptoms may be observed when the plants are irrigated with water −1 −1 that contains 600 mg of sodium L and 900 mg of chloride L and has an EC of 2.1 dS/m. iw Tolerant: No apparent salt stress symptoms may be observed when the plants are irrigated with water −1 −1 containing 200 mg of sodium L and 400 mg of chloride L . Moderate: Less than 10% of symptoms develop when the plants are irrigated with water containing 200 −1 −1 mg of sodium L and 400 mg of chloride L and having an EC of 0.9 dS/m. iw Sensitive: More than 20% of the leaves may develop symptoms when the plants are irrigated with water −1 −1 containing 200 mg of sodium L and 400 mg of chloride L and having an EC of 0.6 dS/m. iw c The definitions of soil salinity tolerance are as follows: −1 Highly tolerant: Permissible soil ECe greater than 6 dS m , −1 Tolerant: Permissible soil ECe greater then 4 and less than 6 dS m , −1 Moderate: Permissible soil ECe greater than 2 and less than 4 dS m , and −1 Sensitive: Permissible soil ECe less than 2 dS m . V­9

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Epipremnum sp. Schott. Pothos Moderate Moderate Ficus pumila L. Creeping fig Highly tolerant Tolerant Hedera canariensis Willd. Algerian ivy Highly tolerant Tolerant Hedera helix L. English ivy Moderate Moderate Hylocereus undatus Britton & Rose Night blooming cereus Moderate Moderate Ipomoea pescaprae R. Br. Railroad vine Highly tolerant Tolerant Ipomoea stolonifera Gmel. Seafoam morning glory Highly tolerant Tolerant Philodendron williamsii Hook. Philodendron Moderate Moderate Passiflora incanata L. Passion flower Sensitive Sensitive Salvia farinacea Benth. Mealycup sedge Sensitive Sensitive Tecomaria capensis Spach. Cape honeysuckle Tolerant Tolerant Trachelospermum jasminoides Lem. Star jasmine Tolerant Tolerant a Data in the table adapted from Wu and Dodge, 2005 (in press). b Tolerances of salt spray are defined by the degree of salt stress symptoms developed in the leaves of the plants and the salt concentrations in the irrigation water as follows: Highly tolerant: No apparent salt stress symptoms may be observed when the plants are irrigated with water −1 −1 containing 600 mg of sodium L and 900 mg of chloride L and having an EC of 2.1 dS/m. iw Tolerant: No apparent salt stress symptoms may be observed when the plants are irrigated with water −1 −1 containing 200 mg of sodium L and 400 mg of chloride L . Moderate: Less than 10% symptoms may be observed when the plants are irrigated with water −1 −1 containing 200 mg of sodium L and 400 mg of chloride L and having an EC of 0.9 dS/m. iw Sensitive: More than 20% of the leaves may develop symptoms when the plants are irrigated with water −1 −1 containing 200 mg of sodium L and 400 mg of chloride L and having an EC of 0.6 dS/m. iw c The definitions of soil salinity tolerance are −1 Highly tolerant: Permissible soil ECe greater than 6 dS m , −1 Tolerant: Permissible soil ECe greater then 4 and less than 6 dS m , −1 Moderate: Permissible soil ECe greater than 2 and less than 4 dS m , and −1 Sensitive: Permissible soil ECe less than 2 dS m . V.B.2. Other Sources of Information Literature regarding the response of plants to salinity has accumulated so rapidly over the years that a comprehensive bibliography is needed to help search for key references. Fortunately, L. E. Francois and E. V. Maas of the U.S. Salinity Laboratory assembled such a bibliography in 1978. It contains 2,350 literature citations from 1900 to 1977, including citations for papers that describe the effects of salt and boron on whole plants. Key phrases for each citation include plant name, experimental materials and methods, treatments and variables evaluated, and results or data obtained. The bibliography has four sections, one listing common plant names, another listing botanical names, another describing treatments, and yet another organized by results. An updated version of this bibliography that currently includes over 6,200 literature citations exists on the Salinity Laboratory’s website at www.ars.usda.gov/Services V­13

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/docs.htm?docid=8908. It is available to everyone, with no password needed to access it, as of 2006. Researchers at the Salinity Laboratory have written a number of key papers over the years. In one of the earliest papers, “Salt Tolerance of Ornamental Shrubs and Ground Covers” (Bernstein, Francois, and Clark, 1972), the authors describe their experiments on 25 species of plants salinized with sodium chloride and calcium chloride. They discovered that overall salt tolerance does not correlate well with tolerance to injury by chloride or sodium (specific ions). They also concluded that survival of a plant under highly saline conditions is not necessarily a good indicator of overall salt tolerance. The paper includes several tables and one illustration comparing the salt tolerances of various shrubs and ground covers. Another key reference by Salinity Laboratory researchers is “Salt Tolerance of Ornamental Shrubs, Trees, and Iceplant” (Francois and Clark, 1978). As with the earlier study, the researchers artificially salinized plants with combination of sodium chloride and calcium chloride salts in the water or soil. They evaluated 10 species of shrubs, 2 species of trees, and 4 species of iceplant. Tolerant varieties were reported to include Texas sage (Leucophyllum frutescens), brush cherry (Syzygium paniculatum), Aleppo pine (Pinus halepensis), croceum iceplant (Hymenocyclus croceus), purple iceplant (Lampranthus productus), rosea iceplant (Drosanthemum hispidum), and white iceplant (Delosperma alba). Those species were affected little, if at all, by soil with salinities as high as an ECe (electrical conductivity of the saturated soil paste extract) of 7 dS/m. Sensitive species included glossy abelia (Abelia grandiflora), photinia (Photinia fraseri), Oregon grape holly (Mahonia aquifolium), and Pyrenees cotoneaster (Cotoneaster congestus). Each of those was severely damaged, or killed, when the ECe measured 4 dS/m. Another important finding by these researchers was that leaves typically were injured only at levels of salinity that suppressed growth by 50% or more. Another pertinent reference by Salinity Laboratory researchers is “Salt Tolerance of Plants” (Maas, 1986). In that journal article, Maas examined the salt tolerance of both crops and ornamental plants, including the criteria for establishing salt tolerance, the factors that influence the salt tolerance of plants, and the relative salt tolerances for herbaceous crops, woody crops, and ornamentals in a series of five tables. Maas pointed out that susceptibility to foliar injury varies considerably among species and depends more on leaf characteristics and the rate of absorption of water than on tolerance of soil salinity. Maas examined the effects of chloride, sodium, and boron on both crops and ornamental plants and provided several tables listing sensitivities of plants to chloride, sodium, and boron. V­14

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The Salinity Laboratory’s parent organization, the U.S. Department of Agriculture, published a series of leaflets known as Home and Garden Bulletins during the 1960s and 1970s. One of those, the leaflet titled “Reducing Salt Injury to Ornamental Shrubs in the West” (Home and Garden Bulletin No. 95), describes how salinity affects plants, outlines how to diagnose salt injury, and presents a few strategies for coping with salinity (Bernstein, 1964). This leaflet is available at certain libraries: visit www.worldcatlibraries.org on the Internet, click on “Try a search,” and enter the leaflet’s author and title. The mentioned leaflet has been superseded by another one in the series, “Salt Injury to Ornamental Shrubs and Ground Covers” (Francois, 1980), which includes a table showing the relative tolerances of 41 different trees, shrubs, and ground covers. A PDF of this leaflet can be downloaded from the Internet at www.agnic.msu.edu/hgpubs/modus/morefile/hg231_80.pdf. Though both leaflets were written in earlier decades, they contain pertinent general information. Bernstein (1980) examined the effects of salinity on fruit trees, such as apple, plum, prune, apricot, and almond, which are occasionally used in landscapes. He relates that the relative importance of osmotic effects and specific ion effects on inhibiting plant growth varies widely, depending on the species. He further states that the yields of some species of fruit tree are relatively unaffected by elevated levels of chloride and sodium ions, even when the leaves are severely injured. However, the yields of certain other species of fruit trees are greatly affected by injuries related to chloride or sodium toxicity. Bernstein outlines several other conclusions, too. First, most fruit trees used as crops are salt sensitive. Second, if the salt tolerance for a particular type of fruit tree tends to vary, it is mainly because different varieties or rootstocks absorb toxic ions at different rates. Third, although salinity generally impairs the quality of fruit, in certain cases it can be beneficial to the fruit quality. Fourth, for sprinkler­irrigated trees, uptake of chloride or sodium by wetted leaves can cause severe leaf burn. And fifth, irrigating infrequently, which is often recommended for ornamental trees and shrubs, can accentuate the effect of salinity on fruit trees. The book Abiotic Disorders of Landscape Plants: a Diagnostic Guide (Costello et al., 2003) provides useful guidelines for assessing the salt tolerance of a plant and diagnosing plant­ related problems. The authors list the salinity tolerances and boron tolerances of 610 landscape plants in a table in that book. Entries are listed within categories (shrub, tree, palm, ground cover, vine, herbaceous plant, and turfgrass) and are sorted alphabetically by botanical or scientific name. The list is useful for comparing species and for discovering the salt tolerance or boron tolerance of a particular plant already chosen for a landscape. The authors also provide a table of the same plants sorted according to salt tolerance, as well as a table sorted according to boron V­15

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tolerance, with each entry appearing in one of three columns: high, moderate, or low tolerance. These tables are helpful when one is seeking a particular plant to satisfy a known salt tolerance or boron tolerance. Abiotic Disorders of Landscape Plants: a Diagnostic Guide provides several other useful types of information. One table in the book lists 12 different common fertilizers and the relative salinity of each. Another table in the book displays the salt content of seven kinds of commercially available organic soil amendments, including, for example, chicken manure, steer manure, peat, and redwood compost. Another of the book’s tables provides guidance for readers who need to interpret chemical data resulting from laboratory tests of soil, water, or plant tissue. Yet another table in the book lists the methodology and criteria used in evaluating the salinity and boron tolerance data for another of the book’s tables. Still another table provides a summary of salt­related problems. Equally useful, if not more so, is information in Chapters 1, 4, 5, and 6 of the aforementioned book on a structured process for diagnosing plant problems caused by salinity or other abiotic agents. Chapter 6 illustrates the process by outlining six case studies. Salt tolerances for 18 species of eucalyptus—often used in California’s landscapes due to their adaptability to the climate, their ability to tolerate little to no irrigation, their relative lack of natural pests, and their fairly high rate of growth—are included in the aforementioned book on abiotic disorders of landscape plants (Costello et al., 2003). A list of 60 species of eucalyptus, plus numerous species of casuarina, acacia, and other Australian shrubs and small trees, appears in an appendix of a book published by the UN Food and Agriculture Organization (Tanji and Kielen, 2002). The list of salt­tolerant plants originated from the Australia Department of Agriculture’s farm­revegetation project as part of its sustainable rural development program in 1998. Many books have been published over the years to help people choose landscape trees, shrubs, and ground covers for California’s cool, marine coastal climates and its dry, warm inland climates. Many focus on water­conserving plants because minimizing water usage continues to be one of California’s perennial challenges. Very few of the available books contain information about choosing salt­tolerant plants for those same California climate zones. One book that does, by Perry (1981), provides not only a list of plants tolerant of saline soils but also a list of those that do well in the presence of salt spray. Table V.B.2.1 in this chapter, excerpted and adapted from the lists in Perry’s book, displays the relative salt tolerance of 36 species of shrubs and trees that are well adapted to the climatic zones of the Los Angeles and San Diego areas. V­16

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A number of websites contain helpful information. Currently, the following relevant links are active: · www.edis.ifas.ufl.edu/EP012 At this site of the University of Florida’s Institute of Food and Agricultural Sciences, there are two fairly extensive tables that list the salt tolerances of a number of trees, shrubs, ground covers, vines, and grasses recommended by the institute for landscapes in northern Florida and for southern portions of the state. Many species listed are popular elsewhere in the United States, including California. · www.denverwater.org At this website of Denver Water, Colorado’s largest water utility, click on the side heading “Recycled Water” and then click on the hyperlink “Effects of Recycled Water on Trees and Shrubs” that subsequently emerges on the main window for a number of tips for keeping trees and shrubs healthy when one is irrigating them with recycled water. · www.sanjoseca.gov/sbwr/Landscape/GuidePlantList.htm This section of the website for the city of San Jose, Calif., has a list of locally available plants for landscapes found to be compatible with irrigation by local recycled water. The list includes 47 species of trees, 29 species of shrubs, 10 species of ground covers, 3 species of vines, 7 species of perennials, and 13 species of native grasses. The vast majority are relatively common varieties that are popular for landscapes elsewhere in California. In light of the ever­changing and ephemeral nature of websites and their links, the aforementioned may or may not continue to be active. In any case, a search engine can be used to discover alternate relevant links. V.C. Salt Tolerance of Floricultural Species Beginning over 50 years ago, researchers at the University of California–Los Angeles, the U.S. Salinity Laboratory in Riverside, and the Metropolitan Water District in La Verne evaluated the salt tolerance of many agronomic and horticultural species. Their legacy—salt tolerance ratings assigned to a number of species and the recommendations for soil, plant and irrigation management practices—is still valid and pertinent today. It should be noted, however, that some varieties and cultivars of major crops have changed and that in some cases there can be significant varietal differences in salt tolerance. This finding is particularly true with perennial crops where rootstock, as well as scion, varieties have changed over the years. V­17

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The work of earlier researchers indicated that waters containing 500 parts per million (ppm, or mg/L) of total dissolved solids (TDS) are likely to reduce the growth or cause leaf burn only for the most salt­sensitive plants or for plants grown either in poorly suited soil, along with unfavorable temperature, sunlight, or humidity or with inappropriate irrigation management practices (Pearson, 1949). They determined that waters containing 800 to 1,000 ppm of TDS also may be used without risk, provided that the kinds of salts contributing to salinity (e.g., sodium, chloride, and sulfate) are considered. Most types of fuchsia (Fuchsia spp.), camellia (Camellia spp.), and rhizomatous begonia (Begonia spp.), for example, grow well in waters of 800 ppm of TDS if sulfate is the principal anion. Yet the same water can cause problems for certain varieties of azaleas and for the Rex begonia. These earlier researchers also found that saline waters dominated by chloride may cause unsightly leaf burn, particularly with sprinkler irrigation. In the late 1940s, researchers found that calcium­dominated saline waters seemed less detrimental to the growth of plants than did waters containing high concentrations of sodium. Their work suggested that plants may be adversely affected by interactions or imbalances of ions, either in the plant, in the water, or in the soil (Hayward and Wadleigh, 1949). For example, levels of calcium that meet the nutritional requirements of plants not subjected to sodium­based salinity may be inadequate for plants that are exposed to high levels of sodium (Hayward and Bernstein, 1958). Water in the soil that is dominated by sodium not only reduces the availability of calcium but also reduces the mobility and transport of calcium to actively growing tissues. Salinity­ induced nutritional disorders may result from the effects of sodium­dominated salinity on nutrient availability, as well as on the uptake, transport, and partitioning of competitive ions within the plant. In the 1940s and 1950s, researchers examined the effects of specific ions such as boron, chloride, and bicarbonate in soils and irrigation waters on the health of floral species. Azaleas (Rhododendron spp.), for example, were found to be relatively sensitive to nutritional imbalances, and even with only slightly saline conditions, calcium deficiency was induced by bicarbonate in the irrigation water (Lunt et al., 1956). Researchers reported that floral species typically respond to salinity by growing less: the length and weight of flowering stems were reduced, or flowers were fewer or smaller. Boron, however, was less detrimental than salinity to the number, size, length, and width of flowering stems of azalea and gardenia (Gardenia spp.; Lunt et al., 1957), carnation (Dianthus caryophyllus; Lunt et al., 1956), China aster (Callistephus chinensis; Kohl et al., 1957), gladioli (Gladiolus spp.; Kofranek et al., 1957), and poinsettia (Euphorbia pulcherrima; Kofranek et al., 1956). Once the boron tolerance limits for the species were V­18

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exceeded, injury was characterized by interveinal chlorosis, marginal leaf scorch, and finally, leaf abscission. Refer to Table V.C.1 for boron tolerance limits of selected floral species. Some researchers in the 1960s and later conducted salt tolerance trials in which they used a single salt, generally sodium chloride, as the salinizing agent. Other researchers, however, have + + 2+ recommended using saline water with sodium/(sodium + calcium) ratio, i.e., Na /(Na + Ca ), in the range of 0.1 to 0.7 in experimental studies, as this recommendation better reflects the ion ratios in irrigation water or in the water in the soil for most horticultural crops (Pearson, 1949; Bernstein, 1975). The uncharacteristic salinizing composition of the former may induce ion imbalances that contribute to calcium­related physiological disorders in certain crops (Shear, 1975; Sonneveld, 1988). Furthermore, the use of single­salt solutions in salt tolerance experiments may result in misleading and erroneous interpretations of a plant’s response to salinity. Grattan and Grieve (1999) examined the relationship between a horticultural crop’s mineral nutrients and its salinity tolerance. They reviewed the literature that pertains to salinity and mineral nutrition, particularly nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and boron, and briefly examined the potential interactions between certain micronutrients— copper, iron, manganese, molybdenum, and zinc—and salinity. They concluded that a multiplicity of salinity­nutrient interactions occur simultaneously for many types of plants and that whether those interactions ultimately affect the plant as measured by yield, quality, size or elongation, etc. depends on the levels of salinity, the composition of salts, the species, the nutrients, and a host of other environmental factors. Even under nonsaline conditions, significant economic losses have been linked to inadequate calcium nutrition of horticultural crops. A number of factors can influence the amount of plant­available calcium, including the total supply of calcium, the nature of the counter­ions, the pH of the substrate, and the ratio of calcium to other cations in the irrigation water (Grattan and Grieve, 1999). Calcium­related disorders may even occur in plants grown on substrates where the calcium concentration appears to be adequate (Pearson, 1949; Bernstein, 1975). Symptoms indicating nutritional deficiency are generally caused by differences in calcium partitioning to the growing regions of the plant. All parts—leaves, stems, flowers, and fruits— actively compete for the pool of available calcium, and each part independently influences the movement of calcium. Organs that transpire more actively are likely to have the highest concentrations of calcium. In agricultural crop plants that consist of large heads enveloped by outer leaves, such as cabbage and lettuce, excessive transpiration by the outer leaves diverts calcium from the rapidly V­19

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growing embryonic plant tissue (Bangerth, 1979). A deficiency of calcium manifests as internal browning in the younger tissues of cabbage and lettuce and as “blackheart” in celery. Calcium deficiency may also occur in reproductive tissues and cause decreases in quality such as “blossom end rot” of tomato, melon, and pepper; “soft nose” of mango and avocado; and cracking and “bitter pit” of apple. Artichokes grown under arid, but nonsaline, conditions can exhibit calcium deficiency, with injury appearing as necrosis of inner bracts (Francois, 1995). Horticultural crops that are susceptible to calcium­related disorders without salinity become even more so under saline conditions. As the concentration of salt in the root zone increases, the plant’s requirement for calcium also increases (Bernstein, 1975). At the same time, the uptake of calcium from the substrate may be depressed because of ion interactions, chemical precipitation, and increases in ionic strength (Grattan and Grieve, 1999). When these susceptible crops are also challenged by salinity, their market quality can decline significantly. Very little information is available on the differential partitioning of calcium and any resulting patterns of injury in floricultural species. Certain varieties of Asiatic hybrid lilies are susceptible to calcium­related disorders, whereas others are immune. Injury on “Star Gazer,” “Acapulco,” and “Muscadet” manifests as necrosis of the upper leaves (Chang et al., 2004) and on “Pirate,” as white­gray cross bands on the leaves, as well as tip burn (Berghoef, 1986). The varieties “Alliance” and “Helvetia” appear to be resistant to the disorder (Chang et al., 2004). Poinsettia (Euphorbia pulcherrima) also exhibits variety­dependent susceptibility to calcium deficiency, with injury usually appearing as marginal necrosis of the bracts. Wissemeier (1993) demonstrated that “Angelika” and “Supjibi” were sensitive. In contrast, injuries do not appear to occur in the varieties “Diva Starlight” and “Lilo.” The effect of salinity on the sensitivity of floral crops to calcium­related disorders has not been widely explored. One study, however, was conducted with poinsettia, a moderately salt­ tolerant crop (Cox, 1991; Dole and Wilkins, 1999). No visible symptoms associated with excess fertilizer salinity were observed in “Red Sails” poinsettia (Cox, 2001) or “V−14 Glory” poinsettia (Ku and Hershey, 1991), although measurements of EC revealed that salinity levels in the root zone exceeded the satisfactory range for the crop (Hartmann et al., 1988). Other information on the salt tolerance of floral species results from studies of the responses of plants to chloride­dominated saline irrigation waters. Such water typically contains both sodium chloride and calcium chloride. A few researchers evaluated the salt tolerance of floral crops by using irrigation waters prepared to simulate recycled or saline waters typical of a specific location or site. Dutch growers often use solutions with compositions of salts adjusted to the average found in surface waters in the western Netherlands (Bik, 1980; Sonneveld, 1988). V­20

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Saline waters (EC = 2.5 to 4.5 dS/m) from local wells in Israel continue to be used successfully for growing floral species on over 700 ha throughout the Negev Desert (Shillo et al., 2002). Arnold and fellow researchers (2003) demonstrated that recycled runoff from a plant nursery and water from a constructed wetland were suitable for irrigating certain bedding plants and flowers. Recent floriculture research at the U.S. Salinity Laboratory involved the use of artificial waters specially prepared to mimic three waters used for irrigation in California: the sodium­ and sulfate­ dominated drainage effluents from the San Joaquin Valley, various concentrations of Colorado River water, and groundwaters affected by seawater intrusion along the California coast (Grieve et al., 2005; Carter et al., 2005; and Grieve et al., 2006). An important caveat to bear in mind is that research on the salt tolerance of floricultural species continues to be largely devoted to providing information useful for helping commercial floricultural growers maintain the productivity, quality, and profitability of their plants. The standards of quality for plants in landscapes are far less stringent. For example, because exposure of a plant to salinity generally decreases the length of the stems and the number of florets—two major determinants of quality in commercial flowers—growers of floricultural crops are likely to use the highest quality of water available to maximize the plant’s height and number of blooms. However, a slightly shorter flowering plant with somewhat fewer florets would be aesthetically acceptable for use in a landscape—as long as its overall health remains uncompromised, its stems are robust, its leaves and flowers remain true to color, and its flowers and leaves sustain no visible salt injury. Take the specific example of two species of statice grown to be sold as flowers, Limonium perezii and L. sinuatum, which complete their life cycles in water saltier than seawater (Aronson, 1989). To discover if either could produce marketable cut flowers at lower salinities, both species were grown under irrigation with waters ranging from 2 to 30 dS/m (Grieve et al., 2005). Both species of statice flowered and set seed in all treatments, but their height decreased consistently and significantly as salinity increased, with plants receiving the most saline treatment growing only one­third as tall as those irrigated with nonsaline waters. However, even under severe salt­related stress, both produced healthy plants with attractive foliage and colorful flowers on sturdy, albeit short, stems. The salt tolerance of both species for use as marketable cut flowers is rated as “low” based on stem length (Farnham et al., 1985), but for use in a landscape, they would fall in the “very tolerant” category. It should also be noted that the effects of salinity on floral crops are not always adverse. Salt­related stress can beneficially affect the yield, quality, and disease resistance of a plant. In some instances, the uptake and accumulation of salinizing ions stimulates growth. Cabrera (2001) and Cabrera and Perdomo (2003) observed a positive correlation between relatively high leaf­ V­21

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chloride concentrations (0.45%) and dry weight for container­grown rose (“Bridal Pink” on Rosa manetti rootstock). Yield and quality were unaffected. Salinity imposed early in the life cycle of some cut­flower species tends to limit vegetative growth with favorable results. Salinity­induced reduction in the length of leaf­supporting stems may be beneficial in chrysanthemum, where tall cultivars are treated with growth regulators to keep the plants compact and short. While plant height is often reduced by moderate salinity, the length of time to maturity and the size of developing floral buds generally remain unaffected by stress (Lieth and Burger, 1989). Application of salinity after some optimal period of vegetative growth tends to enhance reproductive growth and often improves quality. Shillo and coresearchers (2003) reported that salinity imposed on Eustoma grandiflorum during its final stages of vegetative growth resulted in significant increases in the number of flowers and in stem weight and diameter. Another benefit of salt treatment was the production of more compact flower clusters, the compactness of which prevents developing buds from drooping. Similar positive effects have been noted with carnation. Salt­related stress during its early reproductive growth resulted in shorter, more robust flower­ bearing stalks with larger developing buds (Baas et al., 1995). Some of the significant varietal differences in salt tolerance reported for cut­flower crops (Table V.C.2) may be due to differences in climate, nutrition, composition of the salinizing medium, and the duration of exposure to salinity. These differences become very important in selecting plants for landscapes irrigated with recycled waters. In trials conducted under nearly identical cultural conditions, Sonneveld and coresearchers (1987, 1999) reported that the carnation cultivar “Beauty” was significantly more tolerant of soil salinity than were either “Scania” or “Nora Barlo.” In the same study, the hybrid lilies “Star Gazer” and “Connecticut King” both produced lighter­weight flowers when the salinity in the soil extract exceeded 1.2 dS/m. Also, the lilies produced 9.6 and 4.6% fewer flowers, respectively, with each unit increase in salinity. Additional information regarding varietal differences in salt tolerance for selected cut flowers is included in Table V.C.3. The parameters used to assess the salt tolerance of cut flowers need to be considered to accurately assign a tolerance category to a species. Generally, flower quality is less sensitive to salinity than is vegetative growth. For example, once the threshold of “Fabiola” gerbera (Gerbera jamesonii) is exceeded, yield based on the number of flower­bearing stalks per plant declines 17% for each unit increase in salinity, but the diameter of the flowers is relatively insensitive, declining only 3% per unit increase. Likewise, the number and weight of flowering stalks in Anthurium spathes are more affected by salinity than are the diameter of its flowers. The salt tolerance of the poinsettia variety “Barbara Ecke Supreme” is higher when the rating is based on V­22

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V.D. Salt Tolerance of Turfgrasses The quality of a turfgrass stand is the net result of inherent genetic characteristics of the particular species being grown and the interactions of climate, pests, and the soil. In arid and semiarid regions where rainfall is insufficient to leach salt out of the root zone, excessive amounts of soluble salts may accumulate in the root zone. This phenomenon can impose limits on the production or the management of quality turf (Carrow and Duncan, 1998; Marcum, 2006). Salinity­related stress on turfgrasses is also a serious problem near the seacoast, both because the concentration of salt in the air typically is higher than that found inland and because shallow water tables may be unusually saline. Wherever salinization of soils occurs, it is a continuous process resulting from various combinations of these factors: insufficient rainfall, inadequate irrigation, poor drainage, irrigation with water of poor quality, and the upward movement of salts from saline shallow groundwater. As a general rule, if the amount of water applied to the soil (irrigation plus natural precipitation) exceeds evapotranspiration, salt moves downward. Conversely, if evapotranspiration exceeds the amount of water applied, salt movement is upward. In the latter case, salt drawn to the soil surface gradually accumulates to levels toxic to turfgrasses. Depending on the salinity tolerance of the turfgrass grown, full stands of grass can sometimes be established at low or moderate levels of soil salinity. Turfgrass growth in highly saline soils, however, is restricted (Carrow and Duncan, 1998). The symptoms of salinity­related stress in turfgrasses are likely to vary somewhat, because existing salt can result in osmotic stress (physiological drought), nutritional imbalances, toxicity, or a combination of these maladies. In general, however, the following symptoms are associated with turfgrass grown under saline conditions: · Turf is likely to appear blue­green or light bright­green in color during the early stages of salt stress. This coloration is followed by irregular shoot growth. · Necrotic spots may develop on leaves if toxicity from a specific ion (such as boron) occurs. · As salinity­related stress increases, the shoots increasingly wilt and become progressively darker green. · Higher levels of salinity cause burning of the tips of leaves, with the burn eventually extending downward toward the entire leaf surface. At this level, shoot growth is greatly reduced and turfgrass is stunted. As salinity­related stress increases, leaves generally become finer textured and the growth of roots is stunted, often resulting in V­28

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shallow roots. If corrective steps are not taken, the growth of grass will be minimal, the density of shoots will decrease, and individual plants will die, thinning the stand. The extent of salt uptake and its consequent effects on the growth of turf are directly related to the concentration of salt in the soil water. Growth of most turfgrasses is not significantly affected by salt levels below an ECe of 2 dS/m. In soils with salt levels of more than 2 dS/m, the growth of most turfgrasses is gradually restricted. Some notable exceptions, however, would include bermudagrass and seashore paspalum, which can tolerate soil salinities greater than an ECe of 10 dS/m. Due to pronounced differences among turfgrass species and cultivars in their tolerance to both individual salt ions and total salinity, each turfgrass must be individually evaluated with regard to a specific type of soil salinity. · Higher levels of salinity cause burning of the tips of leaves, with the burn eventually extending downward toward the entire leaf surface. At this level, shoot growth is greatly reduced and turfgrass is stunted. As salinity­related stress increases, leaves generally become finer textured and the growth of roots is stunted, often resulting in shallow roots. If corrective steps are not taken, the growth of grass will be minimal, the density of shoots will decrease, and individual plants will die, thinning the stand. Due to many interacting factors, the “absolute” salinity tolerance of a turfgrass species cannot be determined. However, different turfgrasses can be compared, with relative salt tolerance given in terms of the acceptable salt content of the soil root zone, expressed as the ECe of soil water extract. Table V.D.1 (Harivandi et al., 1992; Marcum, 1990; Marcum, 1999) is a general guide to the salt tolerance of turfgrass species (substantial differences in salt tolerance exist among cultivars within species) and shows, for example, that Kentucky bluegrass (Poa pratensis) −1 tolerates soil salinity at ECe levels up to 3 dS m . As the table indicates, soils with an ECe below −1 3 dS m are considered satisfactory for growing most turfgrasses. Soils with an ECe above 10 dS −1 m successfully support only highly salt­tolerant turfgrass species. Salt tolerances of warm­ season and cool­season turfgrass cultivars, given in terms of both top growth and root growth, have been summarized by Carrow and Duncan (1998). Much work has been done in screening existing cultivars or ecotypes for salinity tolerance, including these turfgrass species: Agrostis stolonifera (Marcum, 2001), Buchloe dactyloides (Wu and Lin, 1994), Cynodon spp. (Dudeck et al., 1983; Francois, 1988; and Marcum, 1999), Distichlis spicata (Marcum et al., 2005), Festuca spp. (Horst and Beadle, 1984; and Leskys et al., 1999), Lolium perenne (Rose­Frincker and Wipff, 2001), Paspalum vaginatum (Dudeck and Peacock, 1985; Marcum and Murdoch, 1990; and Lee et al., 2004a; 2004b), Poa V­29

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tolerances of such plants. Southwestern Landscaping with Native Plants (Phillips, 1987) provides relative salt tolerances (as well as other horticultural information) for numerous trees, shrubs, and ground covers that are native to southeastern California, Nevada, Arizona, New Mexico, southern Colorado, southern Utah, and western Texas (see Table V.E.1.1). We have excerpted from that book and then consolidated and edited relevant data for those species of plants reported to be natives of California. The result is Table V.E.1.2, which lists 21 different varieties of shrubs, trees, and ground covers that may be useful for landscape projects in southern California. It is important, however, that the plants featured in this table are arid land varieties; therefore, some may not be particularly well suited for landscapes in Los Angeles or San Diego or elsewhere along the southern California coastal plain. Cross­checking these entries against other sources of horticultural information is recommended. In the absence of published quantitative data from controlled experiments or field trials involving the salinity of native plants, qualitative salt tolerance information may prove useful. The key is to collect such information with care and to test the information thoroughly for soundness. One method for qualitatively estimating the salt tolerance of a plant is to infer that if the plant originated in an area where saline soils are common, then that plant may do well in other saline environments. Such reasoning is not without risk, however, because many other environmental factors are important during the establishment and growth of a plant and because one or more of those factors may not match between the plant’s native origin and the desired site. For example, the microclimate where a plant originally thrived in the wild may not match that of the intended landscape even though the salinity of the soil and perhaps various other factors may be similar. Another strategy that might work well is to choose several different desirable native species for your landscaping project and then attempt to research those or similar plants in Costello et al. (2003) or other references that list salt tolerance data for “conventional” ornamental plants. It may be that one or more of the California native plants for which information is sought have already become a somewhat popular plant and that their salt tolerance is listed in one of the aforementioned sources. V­31

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V.F. Sensitivity of Plants to Specific Ions A plant can be salt tolerant yet still be sensitive to and potentially damaged by specific ions. The ions responsible for most of the damage are chloride, sodium ion, and boron. In the paragraphs that follow, some of the effects of chloride, sodium ion, and boron are described. V.F.1 Sensitivity of Trees, Shrubs, Ground Covers, and Floricultural Species At the early stages, symptoms of salinity and specific ion toxicities in plants are often difficult to distinguish from each other. Foliage may be off­color green with yellowing of the tips or margins of leaves. These observed symptoms, however, are of little diagnostic value unless accompanied by chemical analysis for specific ions in the tissue. Chloride Ion The element chlorine is an essential micronutrient for plants. Its common ionic form of − chlorine is chloride (Cl ). Woody species appear to be more susceptible to chloride toxicity than are herbaceous crops. Tolerances of woody species vary among varieties or rootstocks and are associated with the plant’s ability to restrict the accumulation of chloride in the shoot and particularly in the leaves. Tolerances may be significantly improved by selecting varieties or rootstocks that prevent accumulation of chloride. Moderate chloride toxicity in stone fruit trees may cause reduced vigor and no other visible symptoms. More severe toxicity often results in bleached or bronzed leaves and in scorched margins on leaves. In citrus, bronzing from chloride toxicity is difficult to distinguish from the orange mottling caused by boron toxicity. Proper selection of rootstock helps to avoid the effects of chloride toxicity. Sodium Sodium is not considered an essential nutrient for most plants, yet it does aid the growth of plants at concentrations below the salt tolerance threshold. In water, sodium exists as sodium + ions (Na ). Above the salt tolerance threshold, the sodium ion can have both direct and indirect detrimental effects on plants. Direct effects caused by the accumulation of toxic levels of sodium ions are generally limited to woody species. Symptoms of injury do not usually appear immediately after saline water is applied. Initially, the sodium ion is retained in the roots and basal sections of the trunk. After several years, as sapwood is converted to heartwood, stored sodium ion is released and transported to the leaves, causing the burning of leaves and abscission—the separation of flowers, fruits, or leaves from plants at a special separation layer. V­34

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Indirect detrimental effects of sodium ions include nutritional imbalances and impairment of the soil’s physical condition. The presence of sufficient calcium both in the plant and in the root environment is essential to prevent the accumulation of sodium ions to levels that are injurious to the plant (Maas, 1986). Symptoms of sodium­induced calcium deficiency are weak stems, chlorosis and necrosis of leaves, and leaves distorted by failure to unroll. Boron Boron (B) is an essential micronutrient for plants. For most crops, the optimal concentration of plant­available boron falls within a very narrow range and various criteria have been proposed to define those levels necessary for adequate boron nutrition and yet low enough to avoid toxicity that results in injury and reduced yield (Maas, 1986; and Grieve and Poss, 2000). Boron deficiency is more widespread than boron toxicity, particularly in humid climates. In contrast, boron toxicity is more of a concern in arid environments, where salinity problems also exist (Grattan and Grieve, 1999). As with salt tolerance, the tolerance of a plant to boron varies, depending on the climate, the soil’s conditions, and the variety of the plant. Many of the existing data about boron tolerance were obtained from experiments conducted during 1930 to 1934 by Eaton (1944). These experiments provided threshold tolerance limits for more than 40 different crops. While useful, Eaton’s data cannot be reliably correlated to the corresponding growth of most crops (Maas and Grattan, 1999). Bingham et al. (1985) demonstrated that the response of plants to excess boron can be described by a two­piece linear response model. Threshold and slope parameters for such a model have been estimated for a limited number of crops. Francois and Clark (1979) examined the response of 25 species of ornamental shrub to being irrigated with waters containing either high or low concentrations of boron, 7.5 or 2.5 mg/L, respectively. Boron tolerance was based on reduced growth and deterioration of the plant’s appearance overall (Table V.F.1.1). The salt tolerance of these species had been established in an earlier study (Bernstein et al., 1972), and no correlation was found between the boron tolerances and salinity tolerances of the species tested. Maas (1984) also studied the boron tolerance limits for a variety of ornamental plants (Table V.F.1.2). Symptoms of Boron Toxicity As boron in the root environment increases, characteristic visible symptoms become evident and often sharp boundaries distinctly separate the symptomatic and the unaffected green tissues. Leaves exhibit scorched and necrotic margins, finally dropping prematurely. Chlorosis followed by necrosis first appears at the end of the veins of leaves. In parallel­ veined leaves, such as in grasses and lilies, the necrosis is found at the tips of leaves where the V­35

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veins end. A similar pattern is found in lanceolate leaves, such as those of stock and carnation, where the principal vein terminates at the tip. In such species as geranium, where veins are more radially distributed, boron toxicity appears as an injured zone all around the margin. In leaves with a well­developed network of veins and with many veins ending in areas between principal side veins, such as the leaves of gerbera, aster, and most citrus species, symptoms first develop as interveinal yellow or red spots. As injury progresses, chlorosis spreads to the margins (Oertli and Kohl, 1961). Ranges of boron concentrations in healthy, chlorotic, and necrotic plant tissues are shown in Table V.F.1.3. Necrosis of leaves may be expected when concentrations of boron in plant tissue reach 1,500 to 2,000 ppm on a dry weight basis. Differences in the time necessary for the plants to show symptoms of boron toxicity are apparently not caused by differences in tissue tolerance but, instead, are a function of the rate at which boron accumulates (Oertli and Kohl, 1961). Other symptoms of boron toxicity commonly observed in plants in landscapes include terminal dieback of twigs, necrotic spots on leaves, abnormal forms and textures in leaves, and cracking bark. Necrosis associated with boron is often black and sometimes red such as in eucalyptus, and for most species it is more severe on the older foliage (Chapman, 1966). Characteristic symptoms of boron toxicity in stone fruit trees include reduced bud formation, poor fruit set, and malformed, very poorly flavored fruit (Johnson, 1996). In citrus trees, symptoms often progress from chlorosis and mottling of the leaf tips to the formation of tan, resinous blisters on the underside of leaves (Wutscher and Smith, 1996). V.F.2 Sensitivity of Turfgrasses In addition to overall deleterious effects of salinity, several ions comprising the total salinity may have a direct toxic effect on turfgrasses. The most important of those ions are sodium, chloride, and boron. Sodium The roots of a turfgrass plant absorb sodium and transport it to the leaves, where it can accumulate and cause injury. The leaf symptoms of sodium toxicity resemble those of salt burn. Because salts can be absorbed directly by leaves, irrigation water with a high level of sodium salts can be particularly toxic if applied to turfgrass leaves via overhead sprinklers. Sodium toxicity is often of greater concern with plants other than turfgrasses, primarily because the accumulated sodium in turfgrass is removed each time the grass is mowed. Of the grasses grown on golf courses, annual bluegrass and bentgrass are the most susceptible to sodium phytotoxicity. V­36

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Chloride Besides contributing to the total concentration of soluble salt in irrigation water, chloride may be directly toxic to plants grown on a golf course, park, or other landscape site. Although chloride is not particularly toxic to turfgrasses, many trees, shrubs, and ground covers are quite sensitive to it. Chloride is absorbed by the roots of plants and translocated to leaves, where it accumulates. In sensitive plants, this accumulation leads to necrosis: scorched margins of leaves in minor cases and death of the leaves and abscission in severe cases. Similar symptoms may occur on sensitive plants if high­chloride water is applied via overhead sprinklers, since chloride can be absorbed by wetted leaves as well as by roots. As long as they are mowed regularly, turfgrasses tolerate all but extremely high levels of chloride. Boron Boron is an essential micronutrient for the growth of plants, though it is required in very small amounts. Even at concentrations as low as 1 to 2 mg/L in the saturation extract of soil, it is harmful to most ornamental plants and capable of causing leaf burn. The most obvious symptoms appear as a dark necrosis on the margins of older leaves. Turfgrasses generally tolerate boron better than do most other plants grown in a landscape—even though they are more sensitive to boron toxicity than to either sodium or chloride toxicity. Most turfgrasses may be grown in soils with levels of boron as high as 5 mg/L. V­37

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accumulation in their leaves. This is especially true for salt­sensitive species. Moderately tolerant plants often develop salt injury later—in the fall, for example—when the accumulation of salt in their leaves surpasses threshold levels. For moderately salt­sensitive and salt­sensitive plants, the soil’s salinity and irrigation practices need to be closely monitored and well managed. Any adverse soil conditions inevitably reduce the overall health of plants, causing the plants to become more vulnerable to salt­related stress. Soil with poor structure or impermeable layers can restrict the growth of roots and the distribution of water and salt in the soil. Flooded or poorly drained soils have poor aeration and in some situations can foster the development of a shallow water table. Less fertile, nutrient­deficient soil can reduce the salt tolerance of plants. The salinity of soil in the field is seldom constant and may indeed be highly variable (see Chapters III and IV). Concentrations of salt near the soil’s surface can be nearly equal to that of the irrigation water and many times greater at the bottom of the root zone. If a saline water table exists within 1 m of the surface, and if leaching fractions are low, salts may be transported upward by capillary flow, in which case the highest concentrations of salt will tend to be found at the surface. The salinity of soil also increases between irrigations, due to the transpiration of water withdrawn by the roots and evaporation of water in moist soil surfaces. Water is lost in the vapor form to the atmosphere in both transpiration and evaporation, and soluble salts remain in the soil solution. The growth of plants closely responds to changes in salt concentrations in the root zone because this is where most water absorption is occurring. Modifying the soil’s physical condition and improving management practices can reduce the salt accumulation in the root zone and therefore better sustain plants. The method of irrigation—drip, surface application, or sprinklers—will influence how landscape plants respond to irrigation water of a given salinity. In California, sprinkler irrigation is preferred for most landscapes because it requires less maintenance and is less vulnerable to damage than drip irrigation. Plants irrigated by sprinkling, however, are subject to injury not only from salts in the soil but also from salt absorbed directly through the wetted leaves. Management of the sprinkler irrigation of plants in landscapes can affect the degree of injury to leaves caused by salt deposition. Wherever possible, irrigation should be infrequent and heavy, rather than frequent and light. Slowly rotating sprinklers that allow drying between cycles should be avoided. It is best to sprinkler irrigate at night or in the early morning, avoiding hot, dry, and windy days. Extra management will be needed to irrigate salt­sensitive and moderately salt­tolerant species of plants with recycled water, if salt concentrations in the recycled water exceed the tolerance levels of the species. Such species are particularly vulnerable in the early stages of growth. For example, the young leaves and buds of salt­sensitive and moderately salt­tolerant V­44

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trees are more vulnerable to being sprayed with saline water than are mature leaves. Once the trees grow above the height of the spray, there is less need for this type of sprinkler management, though branches at lower levels may still be exposed to the spray of water and develop symptoms of salt­related stress. In irrigation of salt­sensitive shrubs and ground covers, switching to drip irrigation can help prevent the water from coming into contact with the foliage. However, with drip irrigation, the salinity of the soil needs to be monitored (Chapter IX). When one is designing a new landscape or upgrading an older one, much advantage can be gained by grouping plants of similar salt tolerance in the same area. Each such area can then be irrigated accordingly. Recommendations advanced by researchers for growing ornamental plants and cut flowers with moderately saline waters include the following: · Water more heavily and less often. · Keep the soil as moist as possible without retarding the plant’s growth or creating disease problems. · Use soil containing considerable organic matter. · Select varieties most tolerant of the type of water being used. · Apply slow­release fertilizers as needed to meet the plant’s nutritional requirements, since leaching to control the salinity of the soil may reduce its fertility. · Confirm suspected salt­related injury to a plant before beginning to correct it, as the causes may be unrelated to salinity. For instance, stunting of growth may result from drought, and leaf burn may be caused by drought or toxic amounts of boron. · Judge the suitability of a particular water for irrigation not only by considering its salt content but also by evaluating the manner in which the water is applied, as well as the type of soil to which it will be applied. V.H. Gallery Two color plates accompany the text of this chapter. Plate 1 consists of two pages displaying 14 color photos of salt­damaged plants and leaves. The species and genera illustrated include the following: liquidambar (Liquidambar styraciflua), bottlebrush (Callistemon spp.), bougainvillea (Bougainvillea glabra), cotoneaster (Cotoneaster spp.), crape myrtle (Lagerstroemia indica), eucalyptus (Eucalyptus spp.), hibiscus (Hibiscus spp.), orchid tree (Bauchinia purpura), and xylosma (Xylosma spp.). Plate 2 consists of a single page displaying images of a boron­damaged liquidambar tree. V­45

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Chapter VI. Selecting Plants for Coastal Southern California Landscapes L. Rollins and A. Harivandi VI.A. Selection Based on Zones VI.B. Guides for Selecting Trees, Shrubs, and Ground Covers VI.C. Guide for Selecting Turfgrasses VI.D. Guides Available via the Internet VI.E. Guides about Native Plant Communities V.I.F. Selecting Plants for Certain Types of Landscapes VI.F.1. General Guidelines VI.F.2. Selecting Plants for Golf Courses VI.F.3. Selecting Plants for Playing Fields VI.F.4. Selecting Plants for Parks VI.F.5. Selecting Plants for Medians and Sides of Streets VI.G. Using Guides Cited VI.H. References In this chapter, numerous plant selection guides are described, with a particular focus on guides that pertain to the Los Angeles and San Diego areas, portions of which are served by pipelines conveying recycled water that could be used for landscape irrigation. The goal is to provide information helpful in choosing plants for relatively large landscapes, such as regional parks, golf courses, city parks, cemeteries, and highway medians. Most of the information is equally relevant for selecting plants for smaller landscapes, such as neighborhood parks or street­side plantings. Some of the plant species mentioned in this chapter may be less than ideal for sites irrigated with water of moderate or high salinity. For projects that rely on such water, the site designer should screen candidate species by reading about them thoroughly here and in Chapter V (and in the references cited in both chapters), afterward choosing only those species that are relatively salt tolerant. Likewise, some of the plants listed here may be appropriate only for well­ drained soil. Others may be considered weeds or invasive species in certain circumstances. Careful screening of candidate species is, once again, the best way to ensure a proper match between plant and environment. VI­1

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V.I.A. Selection Based on Zones A plant responds to many different environmental factors, including the following: · Intensity and daily duration of sunlight · Quantity of rainfall and of irrigation water · Quality of available water · Amount and type of nutrients available · Temperature · Physical and chemical properties of the soil · Physical disturbances, such as wind, flooding, or fire · Biotic interactions, such as competition by other plants for space or for sunlight, grazing by plant­eating animals, and diseases caused by various microbes. These various factors, when examined together, can be used to help define a series of geographical and climatological zones or regions for California. This is the approach taken by the authors of some of the more comprehensive plant selection guides described in the following section. The idea is that, by looking at a plant environment map and finding out what zone pertains to a desired project area, one can more reliably determine which species of plants will survive and prosper in the area. VI.B. Guides for Selecting Trees, Shrubs, and Ground Covers Seven books are described below, each of which contains information that is helpful when selecting trees, shrubs, and ground covers. Three of the books cover both native and nonnative species. The other four are specifically about species native to California. Trees and Shrubs for Dry California Landscapes (Perry, 1981) This book is a comprehensive guide to plants for California landscapes, with an emphasis on species that survive with limited water. The book includes lists of plants that tolerate certain problematic situations in landscapes, such as saline spray and alkaline soil. It also includes detailed descriptions, accompanied by photographs, of 360 different species. Some of the species are native to California. Others are natives of other areas in the world that have Mediterranean­like climates similar to California’s: central Chile, South Africa, parts of southern Australia, and, of course, the countries bordering the Mediterranean Sea. VI­2

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Perry reviews the plants in light of their suitability for specific environments. He then relates that choosing plants for a landscape is specialized work and that the plant selection tables and descriptions of plants he provides should be interpreted with caution. He recommends cross­ checking against the lists and information of others. Perry’s book contains four main sections. In the first part, “Regional Plant Environments,” he describes nine macroenvironment zones, shown in Figure VI­1. He also includes a detailed plant selection guide consisting of seven lists: ground covers, small shrubs, medium shrubs, large shrubs, small trees, medium trees, and large trees. Each list contains information about the compatibility of the relevant plants with each of the aforementioned zones. Figure VI­1. Map of plant environments for coastal southern California (after Perry, 1981). VI­3

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Because Perry’s lists represent an invaluable source of information unavailable elsewhere and because his book is now out of print, we have, with his permission, reproduced the lists in this document as Tables VI­1 through VI­7. These tables, when used with the plant environment map, are a highly useful tool for selecting plants during the design of a landscape. The tables and map can be best used in the following stepwise fashion: (1) Using the map, select the appropriate zone or zones for the area of interest. (2) Using the tables, choose plants that seem to fit the project. (3) Cross­check with other relevant guides, such as the Sunset Western Garden Book (2001) to make sure that the selected plants truly are optimal for the project’s local microclimate. (4) Gather details about the selected plants from other books and, if possible, from landscape designers or horticultural experts familiar with the area where the project is located. In the second part of Perry’s book, titled “Planting Guidelines,” he points out that any landscape project consists of various site­specific conditions and design criteria. The goal, he says, is to establish an appropriate planting concept within the constraints of function, aesthetics, costs, resources, and required maintenance. Several different criteria for design are addressed by the various guidelines provided, including the following: planting from seed, planting on slopes, planting for fire safety, and using species native to California. For each such guideline, Perry lists a number of plants that satisfy the stated criterion. For example, in the guideline for fire­safe landscapes, he defines four zones in the landscape and describes how to use those zones to protect a project’s buildings from adjacent fire­prone natural vegetation, such as chaparral. In the third part of the book, titled “Plant Lists for Landscape Situations,” Perry lists plants that can tolerate certain problematic situations that may be encountered in a landscape, such as oak root fungus, alkaline soil, saline spray, and invasive plants. The fourth, final, and most comprehensive part of Perry’s book, titled “Plant Compendium,” consists of text and photographs of 360 different species that have proven useful or popular for California landscapes. VI­4

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Botanical name Common name Native? Coastal Intermediate Coastal Inland Inland (3 = yes) margin valleys foothills valleys foothills Zelkova serrata Sawleaf zelkova • + • • + a Most of the information in the table above is from the book Trees and Shrubs for Dry California Landscapes (Perry, 1981). •,plant is well suited to this type of landscape; +, plant will do well in thi